ALBERT

Description: Abstract Measurements of electrons and ions in Saturn's ionosphere down to 1500 km altitudes as well as the ring crossing region above the ionosphere obtained by the Langmuir probe onboard the Cassini spacecraft are presented. Five nearly identical deep ionosphere flybys during the Grand Finale orbits and the Final plunge orbit revealed a rapid increase in the plasma densities and discrepancies between the electrons and ions densities (Ne and Ni) near the closest approach. The small Ne/Ni ratio indicates the presence of a dusty plasma, a plasma which charge carrier is dominated by negatively charged heavy particles. Comparison of the LP obtained density with the light ion density obtained by the Ion and Neutral Mass Spectrometer (INMS) confirmed the presence of heavy ions. An unexpected positive floating potential of the probe was also observed when Ne/Ni 〈〈 1. This suggests that Saturn's ionosphere near the density peak is in a dusty plasma state consisting of negatively and positively charged heavy cluster ions. The electron temperature (Te) characteristics in the ionosphere are also investigated and unexpectedly high electron temperature value, up to 5000 K, has been observed below 2500 km altitude in a region where electron‐neutral collisions should be prominent. A well‐defined relationship between Te and Ne/Ni ‐ratio was found, implying that the electron heating at low altitudes is related to the dusty plasma state of the ionosphere.

Description: Abstract The Mars Advanced Radar for Subsurface and Ionospheric Sounding onboard the Mars Express spacecraft measures the frequency of local plasma oscillations, which can be used to determine electron densities local to the spacecraft. This paper provides an overview of electron densities in the upper Martian ionosphere, obtained by investigating over 400,000 ionograms, during the course of about 11 years, corresponding to a full solar cycle. The data cover wide latitude and longitude ranges, 180° of solar zenith angle (SZA), and altitudes from about 250 to 1550 km. The electron density profiles show large fluctuations within each orbit and also for any given altitude and SZA range. However, the median electron density is almost constant on the dayside at a fixed altitude range, with the exception of a dip at around 30° SZA, at altitudes between 300 and 600 km. A sudden drop in density is observed as the terminator is approached from the dayside. For a fixed SZA range, the median electron density decreases exponentially with increasing altitude. The high‐altitude scale height is composed of two exponential functions of SZA joined near the ionospheric terminator. The e‐folding height changes between 45 and 214 km from the subsolar point up to 120°, corresponding to effective temperatures between about 165 and 780 K. Solar activity has a clear effect on the median electron densities above 500 km and on e‐folding height. The median electron density is higher during northern winters, as well as above regions of strong crustal fields on the dayside.

Description: Abstract This paper reports the ionospheric response to typhoon Chan‐hom using GPS network data from China, combined with observations of an ionosonde chain. Two MSTIDs were observed around the two landing times of typhoon Chan‐hom, respectively. The MSTID with a phase fronts aligned in the north‐south direction was detected on the east coast of China at 16:50‐18:40 UT on 11 July 2015, and moved westward with a mean horizontal phase velocity of 268 m/s and period of 56 min. Then the concentric MSTID appeared on the northeast of China at 01:50‐05:10 UT on 13 July 2015, and propagated in the radial direction with average horizontal phase velocity of 143 m/s and period of 45 min. There may be some connections between the north‐south aligned MSTID and the electrodynamical coupling of the sporadic E layer and the F layer, while the concentric MSTID may be caused by the concentric gravity waves resulting from the bodyforces related with the typhoon.

Description: Abstract The Global Ultraviolet Imager (GUVI) aboard the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet airglow emissions in the thermosphere. The retrieved altitude profiles of thermospheric neutral density from GUVI daytime limb scan are significant for ionosphere‐thermosphere study. Here, we use the profiles of the main neutral density to derive the total mass density during the period 2002‐2007 under geomagnetic quiet conditions (ap〈=12). We attempt to compare the obtained total mass density with the Challenging Minisatellite Payload (CHAMP) observations, making use of an empirical model (GUVI model hereafter). This GUVI model is aimed to solve the difficulty of the direct comparison of GUVI and CHAMP observations due to their different local times at a given location in a given day. The GUVI model are in good agreement with CHAMP observations with the small standard deviations of their ratios (less than 10%) except at low solar flux levels. The correlation coefficients are greater than 0.9 and the relative standard errors (RSE) are less than 20%. Comparison between the GUVI model and CHAMP observations during solar minimum show a large bias (~30%). The large bias at low solar flux levels might be due to the limitation of F10.7 as an EUV radiation flux proxy and the fitting method. Our results demonstrate the validity and accuracy of our model based on GUVI data against the density data from the CHAMP satellite.

Description: Abstract We report the first comparison of ground and satellite measurements of low‐latitude traveling ionospheric disturbances (TIDs). Three TID events were simultaneously observed by a 630.0‐nm airglow imager and the CHAMP satellite at Kototabang, Indonesia (geographic coordinates: 0.2°S, 100.3°E, geomagnetic latitude: 10.6°S). In 630.0‐nm airglow images of all three events, there are clear southward‐moving structures. Events 1 and 2 are a single pulse with horizontal scales of ~500–1000 km. Event 3 shows five wave fronts with a horizontal scale size of 500–1000 km. All three TIDs are medium‐scale TIDs. Horizontal wavelengths of both airglow intensity at an average emission altitude of 250 km and CHAMP neutral density variations measured at 400 km are estimated by fitting a sinusoidal function to the observed data. The estimated horizontal wavelengths for airglow and neutral density data are 1031 and 880 km for event 1 and 560 and 420 km for event 3, respectively. These values between airglow and CHAMP are comparable, suggesting both instruments are observing the same wave. For event 1, the CHAMP electron density mapped along the geomagnetic field line onto the airglow altitude does not show wave structure similar to the airglow variation. For events 2 and 3, the plasma density did not show wavy structures similar to the waves seen in the airglow image and CHAMP neutral density. These results suggest that the TIDs observed in airglow images are not caused by ionospheric plasma instability but by gravity waves in the thermosphere.

Description: Abstract We study characteristics of ion and electron beams observed during 101 crossings of the near‐separatrix region by ARTEMIS spacecraft in the magnetotail. We found that accelerated ion beams are observed under any level of geomagnetic activity. A duration of earthward moving ion beams is statistically longer (≤ 10 min) than a duration of tailward ion beams (≤ 4 min), which can be due to the transient character of ion acceleration in the vicinity of Near‐Earth Neutral Line (NENL). Energetic characteristics of earthward and tailward ion beams are similar indicating similar acceleration conditions at ion kinetic scales at both sides of an X‐line independently of its location. Conversely, electron velocity distributions observed near magnetic separatrix earthward of the Distant Neutral Line (DNL) differ from those observed tailward of the NENL. Earthward of the DNL a scattered and thermalized electron population without energetic field‐aligned beams is observed near the separatrix. On the contrary, tailward of the NENL field‐aligned electron beams accelerated to a few keVs are detected. These observations show that near DNL the electron scattering and thermalization dominate over the direct acceleration, whereas stronger electric fields in the NENL produce substantial population of field‐aligned keV‐electrons.

Description: Abstract Based on multi‐year observations of ionospheric total electron content (TEC) and atmospheric wind, we develop a new method to interpret the atmospheric contribution to the ionospheric wavenumber‐4 (WN4) structure according to their coherences in annual variations. Firstly, the ionospheric WN4 is extracted from the equatorial integrated TEC (ITEC) using the global ionospheric maps (GIMs). The TIDI/TIMED wind observations were used to deduce the latitudinal symmetric and antisymmetric components of the zonal and meridional DE3 (diurnal, eastward, zonal wavenumber 3), SE2 (semidiurnal, eastward, zonal wavenumber 2) and SPW4 (stationary planetary wave with zonal wavenumber 4). We then develop a regression model to estimate the coupling efficiency, the background diurnal influence and the associated contribution in ITEC WN4. (1) The zonal symmetric DE3 is the most efficient tide in generating ITEC WN4, while the zonal symmetric SPW4 plays a secondary but varied role. (2) The diurnal variation of background ionosphere/thermosphere influence's amplitude is similar to that of the zonal mean ITEC during the day, with a westward phase velocity. (3) The zonal symmetric DE3 resulted WN4 can reach 7% of the zonal daily mean ITEC while the observed total WN4 value is about 10%. The contributions by the zonal symmetric SE2, SPW4 and meridional symmetric SE2 are comparable with respective maxima ~1.5% of the zonal daily mean ITEC. The present results confirm former suggestions that symmetric zonal DE3 is the primary source for ionospheric WN4, and the contributions due to antisymmetric wind components are relatively small in ITEC WN4.

Description: Abstract We have developed a seasonally dependent energy loss model to calculate the zonally averaged production rates of O3+ due to impact of Galactic Cosmic Rays (GCR) in the dayside troposphere of Mars between solar longitudes (Ls) ~ 0o and 360o at low latitudes (2oN, 2oS, 25oN and 25oS), mid latitudes (45o and 45oS) and high latitudes (70oN and 70oS) in the Martian Year (MY) 28 and MY 29. We also represent the seasonal variability of zonally averaged ozone column density obtained from Mars Climate Database (MCD) [Millour et al., 2014] during the daytime. These results are compared with the daytime observations of column ozone made by Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) onboard Mars Express (MEX). At mid‐to high latitudes ozone column density is maximum in northern winter and minimum in southern summer. At low‐to‐mid latitudes (2oN‐S, 25oN‐S and 45oN‐S), the production rates of O3+ represent a broad peak between altitudes 26 km and 45 km in both hemispheres. The peak production rates are increasing up to Ls = 47.5o and then stabilized at about 2.5 x 10‐8 cm‐3 s‐1. At Ls ≥ 47.5o the peak production rate of O3+ starts decreasing until it disappeared after Ls = 127.5o. A major dust storm occurred in MY 28 at Ls ~ 280o in southern latitudes (~25o‐35oS). During the dust storm period, dust opacity, ozone column density and O3+ production rate on the surface of Mars were increased by a factor of ~3.

Description: Abstract The high‐resolution thermosphere‐ionosphere‐electrodynamics general circulation model (TIEGCM) has been used to investigate the response of F2‐region electron density (Ne) at Millstone Hill (42.610N, 71.480W, maximum obscuration: 63%) to the Great American Solar Eclipse on 21 August 2017. Diagnostic analysis of model results shows that eclipse‐induced disturbance winds cause F2‐region Ne changes directly by transporting plasma along field lines, indirectly by producing enhanced O/N2 ratio that contribute to the recovery of the ionosphere at and below the F2 peak after the maximum obscuration. Ambipolar diffusion reacts to plasma pressure gradient changes and modifies Ne profiles. Wind transport and ambipolar diffusion take effect from the early phase of the eclipse and show strong temporal and altitude variations. The recovery of F2‐region electron density above the F2‐peak is dominated by the wind transport and ambipolar diffusion, both move the plasma to higher altitudes from below the F2‐peak when more ions are produced in the lower F2‐region after the eclipse. As the moon shadow enters, maximizes and leaves a particular observation site, the disturbance winds at the site change direction and their effects on the F2‐region electron densities also vary, from pushing plasma downward during the eclipse to transporting it upward into the topside ionosphere after the eclipse. Chemical processes involving dimming solar radiation and changing composition, wind transport and ambipolar diffusion together cause the time delay and asymmetric characteristic (fast decrease of Ne and slow recovery of the eclipse effects) of the topside ionospheric response seen in Millstone Hill incoherent scatter radar observations.

Description: Abstract The plasma sensor on the sixth H‐II Transfer Vehicle (HTV) measured the depletion of electron density during some maneuver operations for attitude or altitude control. In this paper, we first discuss the on‐orbit observation of electron density depletion during the entire lifetime of the HTV. On‐orbit data showed that both altitude and attitude control maneuvers caused the depletion effect. The geocentric coordinates and universal time were also shown to confirm the depletion effect around the HTV by using other techniques, such as total electron content based on the Global Navigation Satellite System (GNSS) network. In the latter part of the paper we discuss the cause of the depletion effect based on past literature. To analyze the interaction between thruster plume gas and ionospheric plasma, computational fluid dynamics simulations were conducted to analyze the number density distribution of molecules around the HTV. It was found that the combination of charge exchange and dissociative recombination possibly caused the depletion of electron density during altitude control maneuvers, due to neutral particles being ejected from the thrusters and a time delay in the reaction response. During attitude control maneuvers, the electrons of ionospheric plasma were pushed away from the plasma sensor by dense neutral clouds.

Description: Abstract The newly published spectra of protons and helium over time directly measured in space by the AMS‐02 experiment for the period 2011 – 2017 provide a unique opportunity to calibrate ground‐based neutron monitors (NMs). Here, calibration of several stable sea‐level NMs (Inuvik, Apatity, Oulu, Newark, Moscow, Hermanus, Athens) was performed using these spectra. Four modern NM yield functions were verified: Mi13 (Mishev et al.,2013), Ma16 (Mangeard et al., 2016), CM12 (Caballero‐Lopez & Moraal, 2012) and CD00(Clem & Dorman, 2000), on the basis of the cosmic‐ray spectra measured by AMS‐02. The Mi13 yield function was found to realistically represent the NM response to galactic cosmic rays. CM12 yield function leads to a small skew in the solar cycle dependence of the scaling factor. In contrast, Ma16 and CD00 yield functions tend to overestimate the NM sensitivity to low‐rigidity (〈10 GV) cosmic rays. This effect may be important for an analysis of ground level enhancements, leading to a potential underestimate of fluxes of solar energetic particles as based on NM data. The Mi13 yield function is recommended for quantitative analyses of NM data, especially for ground‐level enhancements. The validity the force‐field approximation was studied, and it was found that it fits well the directly measured proton spectra, within a few % for periods of low to moderate activity and up to ≈10 % for active periods. The results of this work strengthen and validate the method of the cosmic‐ray variability analysis based on the NM data and yield‐function formalism, and improves its accuracy.

Description: Abstract We study the role of substorms and steady magnetospheric convection (SMC) in magnetic flux transport in the magnetosphere, using observations of field‐aligned currents (FACs) by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). We identify two classes of substorm, with onsets above and below 65° magnetic latitude, which display different nightside FAC morphologies. We show that the low‐latitude onsets develop a poleward‐expanding auroral bulge, and identify these as substorms that manifest ionospheric convection‐braking in the auroral bulge region [Grocott et al., 2009]. We show that the high‐latitude substorms, which do not experience braking, can evolve into SMC events if the interplanetary magnetic field (IMF) remains southwards for a prolonged period following onset. We conclude that during periods of ongoing driving, the magnetosphere displays repeated substorm activity or SMC depending on the rate of driving and the open magnetic flux content of the magnetosphere prior to onset. We speculate that sawtooth events are an extreme case of repeated onsets, and that substorms triggered by northward‐turnings of the IMF mark the cessation of periods of SMC. Our results provide a new explanation for the differing modes of response of the terrestrial system to solar wind‐magnetosphere‐ionosphere coupling by invoking friction between the ionosphere and atmosphere.

Description: Abstract This paper discusses the solar cycle variation of the DE3 and DE2 nonmigrating tides in the nitric oxide (NO) 5.3 μm and carbon dioxide (CO2) 15 μm infrared cooling between 100 and 150 km altitude and +/‐40 deg latitude. Tidal diagnostics of SABER NO and CO2 cooling rate data (2002‐2013) indicate DE3 (DE2) amplitudes during solar maximum are on the order of 1 (0.5) nW/m3 in NO near 125 km, and on the order of 60 (30) nW/m3 in CO2 at 100 km, which translates into roughly 15‐30% relative to the monthly zonal mean. The NO cooling shows a pronounced (factor of 10) solar cycle dependence (lower during solar minimum) while the CO2 cooling does not vary much from solar min to solar max. Photochemical modeling reproduces the observed solar cycle variability and allows one to delineate the physical reasons for the observed solar flux dependence of the tides in the infrared cooling, particularly in terms of warmer/colder background temperature versus smaller/larger tidal temperatures during solar max/min, in addition to cooling rate variations due to vertical tidal advection and tidal density variations. Our results suggest that (i) tides caused by tropospheric weather impose a substantial ‐ and in the NO 5.3 μm case solar cycle dependent‐ modulation of the infrared cooling, mainly due to tidal temperature (ii) observed tides in the infrared cooling are a suitable proxy for tidal activity including its solar cycle dependence in a part of Earth's atmosphere where direct global temperature observations are lacking.

Description: Abstract With the measurements of the Magnetospheric Multiscale (MMS) mission at the magnetopause, we investigated the electron distribution and the whistler waves associated with a series of six ion‐scale flux transfer events (FTEs). Based on the magnetic field signature, each FTE can be divided into the core region and the draping region. In the draping regions of the most FTEs, the low‐energy electrons displayed a bidirectional field‐aligned distribution. The medium‐energy electrons showed a field‐aligned or beam distribution in the leading part, while a pancake distribution was presented for the electrons in the trailing part of the draping region, which has never been reported previously. The close correlation between the pancake distribution and the compression of the localized magnetic field suggests that the pancake distribution may be due to the betatron acceleration. The whistler waves associated with the FTEs were observed and categorized into the lower and upper bands according to the frequency range. The lower‐band whistler waves propagated in variable directions and therefore could be generated locally. The trailing part of the draping region with the electron pancake distribution was considered to be one possible source region. On the contrary, the upper‐band whistler waves were all found in the core region and propagated antiparallel to the magnetic field, and therefore originated from the same source region. The observations confirmed that the FTEs are important channels for the mass and wave transport between the magnetosheath and the inner magnetosphere, and the electron dynamics can be modified during the FTE evolution.

Description: Abstract The period of September 6 – 11, 2017 was an active period in which multiple solar flares and a major geomagnetic storm occurred. The two largest flares, an X9.3 and an X8.2 flares, were a disk flare and a limb flare, respectively. We conducted model simulations and data analysis to examine solar flare effects on the coupled thermosphere and ionosphere (TI) system in connection with flare location effects, and to investigate the occurrences of large‐scale traveling atmosphere disturbances (TADs) due to flares and storms. Soft X‐ray enhancement, which dominates E‐region ionization, is essentially not affected by the location of a flare on the Sun; solar extreme ultraviolet (EUV) enhancement, which dominates ionization above ~ 150 km, is much weaker for a limb flare compared to a disk flare with the same magnitude. Consequently, flare responses in the lower thermosphere and ionosphere E‐region are not affected by flare locations, but above ~ 150 km, the TI system responds more strongly to disk flares than to limb flares. However, our studies show that during the space weather events in September 2017, these flare location effects were masked by other factors including local time and longitude. Large‐scale TADs occurred when there were both flares and storms. The presence of the flares changed the magnitudes and propagation speeds of the large‐scale TADs. However, there was no evidence that large‐scale TADs occurred when there were only flares and not storms, indicating that solar flares alone were not sufficient to excite large‐scale TADs.

Description: Abstract We present results of numerical simulation of quasi‐periodic (QP) extra low frequency/very low frequency (ELF/VLF) emissions performed by using a theoretical model of flow cyclotron maser based on a self‐consistent set of equations of the quasi‐linear plasma theory averaged over oscillations of waves and particles in a geomagnetic flux tube. Calculations were made for a wide range of plasma parameters (i.e., cold plasma density, L‐shell, energetic electron flux, etc.) in order to obtain a statistical relationship between various properties of QP emissions, such as the repetition period, the frequency bandwidth, the frequency drift rate, and the characteristic wave spectral energy density. The theoretical results are compared with the results of a statistical study of QP emissions measured by the DEMETER spacecraft [Hayosh et al., 2014]. The simulation results are in a good agreement with the observation data in the case of reasonable choice of cold plasma density value and its dependence on the QP‐source location (L‐shell). In particular, an increase in the frequency bandwidth of QP VLF waves with increasing central frequency of QP emissions, a decrease in the frequency drift rate of QP elements with increasing repetition period, and a decrease in the characteristic wave spectral energy density with increasing repetition period are confirmed.

Description: Abstract Signals from manmade VLF transmitters, used for communications with submarines, can leak into space and contribute to the dynamics of energetic electrons in the inner radiation belt and slot region. In this study we use ∼5 years of plasma wave data from the Van Allen Probe A satellite to construct new models of the observed wave power from VLF transmitters both as a function of L∗ and MLT and geographic location. Average power peaks primarily on the nightside of the Earth for the VLF transmitters at low geographic latitudes. At higher latitudes the peak average power extends further in MLT due to more extensive periods of night‐time in the winter months. Night‐time power is typically orders of magnitude more than that observed near noon, implying that loss rates from a given VLF transmitter will also maximise in this region. The observed power from any given VLF transmitter is tightly confined in longitude, with the nightside peak power typically falling by a factor of 10 within 10o longitude of the location of the peak signal. We show that the total average wave power from all VLF transmitters lies in the range 3‐9 pT2 in the region 1.3 〈L∗〈 3.0, with approximately 50% of this power emanating from three VLF transmitters, NWC, NAA and DHO38.

Description: Abstract Plasma transport in the rapidly rotating giant magnetospheres is thought to involve a centrifugally‐driven flux tube interchange instability, similar to the Rayleigh‐Taylor (RT) instability. In three dimensions, the convective flow patterns associated with the RT instability can produce strong guide field reconnection, allowing plasma mass to move radially outward while conserving magnetic flux [Ma et al., 2016]. We present a set of hybrid (kinetic ion/fluid electron) plasma simulations of the RT instability using high plasma beta conditions appropriate for the inner and middle magnetosphere at Jupiter and Saturn. A density gradient, combined with a centrifugal force, provide appropriate RT onset conditions. Pressure balance requires only a temperature gradient as the magnetic pressure is constant. Pressure balance is achieved with a temperature gradient in a fixed magnetic field. The three‐dimensional simulation domain represents a local volume of the magnetodisc resonant cavity. Simulated RT growth rates compare favorably with linear theory, where the fundamental mode of the resonant cavity determines the largest (stabilizing) parallel wavelength. We suggest that the perpendicular scale of RT structures is determined by the fundamental mode, which limits growth due to magnetic tension. Finally, we investigated strong guide field magnetic reconnection and diffusive processes as plausible mechanisms to facilitate kinetic‐scale radial transport.

Description: Abstract This paper reports a preliminary result for estimating thermospheric temperature around 155 km from the N2 Lyman‐Birge‐Hopfield (LBH) bands observed by TIMED/GUVI. AURIC model [Strickland et al., 1999] calculations indicate that the intensity ratio in the N2 LBH (0,0) and (1,0) bands at 144.5–145.5 and 141.0–142.0 nm quasi‐linearly depend on N2 rotational temperature. The observed ratios and the AURIC results are used together to specify the thermospheric temperature around 155 km under sunlit conditions. The estimated temperature agrees fairly well with the neutral temperature at 155 km from WACCAM‐X model. The estimated temperature is also higher over the auroral oval and O/N2 depleted regions. Furthermore, meridional wave‐like structures were clearly seen in the derived temperature and were likely caused by traveling atmospheric disturbances.

Description: Abstract A comprehensive study of all superstorms (minimum Dst ≤ −250 nT) occurring during the space age (after 1957) and their interplanetary and solar causes has been performed. Most superstorms were driven solely by the sheath preceding an interplanetary coronal mass ejection (ICME) or by a combination of the sheath and an ICME magnetic cloud. Only one superstorm was driven solely by a magnetic cloud. There were two superstorms caused by “compound events” with two ICMEs. No superstorms in this study were induced by corotating interaction regions. For the interplanetary shocks antisunward of the ICMEs, the shock normal angles were almost all quasi‐perpendicular. Quasi‐perpendicular shocks theoretically cause greater sheath magnetic field intensities than do quasi‐parallel shocks. Larger shock normal angles statistically corresponded to greater superstorm intensities. Ninety percent of the superstorms occurred either near solar maximum or during the declining phase, 8% of the superstorms occurred during the ascending phase, and 2% of the superstorms occurred during solar minimum. Fifty‐four percent of the superstorms were associated with X‐class solar flares, 36% were associated with M‐class flares, and 5% with C‐class flares. All solar flares related to superstorms occurred in active regions, indicating the importance of active regions to superstorms. Most flares were located in the central meridian or slightly west of it as expected. Two superstorms were caused by limb flares, one on the west limb (confirmed) and the other on the east limb (unconfirmed). The confirmed event was a sheath event led by a Mach 4.1 shock.

Description: Abstract Cross‐track ion drifts measured by the Swarm A satellite are compared with colocated line‐of‐sight Super Dual Auroral Radar Network (SuperDARN) velocities in approximately the same directions. More than 200 Swarm A passes over four polar cap SuperDARN radars in the Northern and Southern Hemispheres are considered. Overall, the Swarm‐based velocities are larger than the SuperDARN velocities; the slope of the best fit line to the data is ~0.67. Somewhat stronger differences are found when Swarm A measurements for the entire year 2016 are compared with SuperDARN vector data from global‐scale convection maps. Swarm ion drift data demonstrate known features of the high‐latitude convection patterns, for example, reverse convection cells at interplanetary magnetic field Βz 〉 0. The latitudes of the convection reversal boundary inferred from SuperDARN are found to be in reasonable agreement with those determined from Swarm A and Swarm B, with Swarm‐based latitudes occurring roughly 1° more equatorward, typically.

Description: Abstract Saturation properties of whistler wave instability driven by electron temperature anisotropy in a plasma with two electron components are investigated using particle‐in‐cell (PIC) simulations. Most of previous self‐consistent PIC simulations or quasi‐linear theory used a single bi‐Maxwellian distribution of electrons, which might apply to solar wind or magnetosheath. In the inner magnetosphere, however, there is frequently a cold and dense electron component, coexisting with a hot electron component. In this work, we investigate the relation between temperature anisotropy (A) and parallel plasma beta (β‖) at saturation. Our results agree well with the previous conclusion obtained from linear theory in most cases. However, We show that the inverse relation breaks when β‖ is larger than certain value β‖,lim, beyond which A increases with increasing β‖. We also investigated the dependence of the saturation wave intensity on plasma beta and the initial linear growth rate, and show that the saturation amplitude can be modeled as a function of the maximum initial linear growth rate even in a plasma with two electron components. Our results might be useful to couple the microscopic wave excitation process with macroscopic global energetic electron dynamics and to understand whistler wave excitation in the inner magnetosphere.

Description: Abstract Suprathermal electron bursts (STEBs), characterized by a board energy spectrum and a field‐aligned pitch angle distribution, have been well recognized to be associated with electron acceleration by inertial Alfvén waves and are thus conventionally termed as “Alfvénic aurora.” In this study, we report joint Enhanced‐Polar‐Outflow‐Probe (e‐POP) and ground‐based optical observations of Alfvénic auroras. In particular, we highlight the prominence of 630‐nm red line emissions under low‐energy Alfvénic auroral precipitation. During the event interval, e‐POP traverses two arcs. One bright arc dominated by green line emissions is clearly seen by all optical instruments; it is embedded in upward field‐aligned currents (FACs) yet leaves little imprint on the e‐POP suprathermal electron imager (SEI), likely due to that the precipitation is well above the upper energy limit of SEI. On the other hand, there is a red line arc that is pronounced only in 630‐nm images. Such a red‐line‐only arc is located in a transition from large‐scale upward FACs to downward FACs and is associated with a prominent STEB structure detected by e‐POP SEI. The STEB features an inverse energy time dispersion, namely, that lower‐energy electrons are seen earlier while higher‐energy electrons appear later. The red‐line‐only arc and its separation from the green line arc evolve in a repeatable fashion, each stemming from a poleward auroral intensification (PAI) propagated from higher latitudes. Following each poleward auroral intensification the green line arc progressively moved southward, while the red‐line‐only arc is quasi‐stationary and stayed relatively stable in latitude. We propose tentative interpretations of the above features based upon stationary inertial Alfvén waves.

Description: Abstract Regardless of the numerous significant contributions in the field (e.g., Svensmark, H. & Svensmark, J., 2009, https://doi.org/10.1029/2009GL038429; Svensmark, J., et al, 2012, https://doi.org/10.5194/acpd-12-3595-2012), the impact of cosmic rays on Earth's weather is still a source of misunderstanding among scientists. Chree method of analysis (Chree, 1912, https://doi.org/10.1098/rsta.1913.0003) is one of the commonest tool used in the investigation. But the method has been queried by some publications. Greater number of these critiques (Forbush et al., 1983, https://doi.org/10.1007/BF00145551; Prager & hoenig, 1989, https://doi.org/10.1577/1548-8659(1989)118〈0608:SEAART〉2.3.CO;2) (Forbush et al., 1983; Prager & Hoenig, 1989) point to the test of significance of epoch superposition results. Forbush events are the most widely key event time used in solar Earth's weather investigation. Despite the early indications of Marcz (1997, https://doi.org/10.1016/S1364-6826(96)00076-4) that the result of compositing analysis depends on the Forbush event selection criteria and timing, various conflicting methods of event selection still dominate articles documenting Forbush decrease (FD)‐related correlations. The present submission calls the attention of researchers to the need for a systematic FD event identification with respect to timing and magnitude estimation prior to compositing/correlation/regression analyses. The relationship between program‐selected FDs, World Wide Lightning Location Network (WWLLN) data, and solar/geophysical parameters is tested at different regions of the world. Significant correlations between WWLLN and other parameters were observed at the U.S. latitude band and within the African region.

Description: Abstract During geomagnetic storms and substorms, the magnetosphere and ionosphere are strongly coupled by precipitating magnetospheric electrons from the Earth's plasma sheet and driven by both magnetospheric and ionospheric processes. Magnetospheric wave activity initiates electron precipitation, and the ionosphere and upper atmosphere further facilitate this process by enhancing the value of precipitated energy fluxes via connection of two magnetically conjugate regions and multiple atmospheric reflections. This paper focuses on the resulting electron energy fluxes and affiliated height‐integrated Pedersen and Hall conductances in the auroral regions produced by multiple atmospheric reflections during the 17 March 2013 geomagnetic storm and their effects on the inner magnetospheric electric field and ring current. Our study is based on the magnetically and electrically self‐consistent Rice Convection Model‐Equilibrium (RCM‐E) of the inner magnetosphere with SuperThermal Electron Transport (STET) modified electron energy fluxes that take into account the electron energy interplay between the two magnetically conjugate ionospheres. STET‐modified energy flux in the RCM‐E leads to a significant difference in the global conductance pattern, ionospheric electric field formation, Birkeland current structure, ring current energization and it's energy content, SAPS intensifications and their spatial locations, interchange instability redistribution, and overall energy interplay on the global scale.

Description: Abstract The importance of neutral wave dynamics in the understanding of the upper atmospheric processes is well‐known. Conventionally, optical methods are used to derive information on the neutral wave dynamics by obtaining gravity wave (GW) characteristics. Optical measurement techniques use airglow emissions as tracers to obtain such information that correspond to altitudes from where the emissions emanate. However, in this paper, we describe a method using radio wave measurement technique (digisonde) to obtain information on the neutral GW behaviour. It involves monitoring of variations in the heights of isoelectron densities as a function of time, and their phase shifts, if any, to derive vertical propagation speeds and scale sizes of GWs. The daytime values of GW time periods, vertical phase speeds, and vertical scale sizes obtained for the duration of 16‐21 May 2015 are in the range of 1.47 ± 0.05 to 2.64 ± 0.07 h, 30.06 ± 4.35 to 45.69 ± 11.84 ms‐1, and 183.21 ± 39.23 to 393.07 ± 66.38 km, respectively. Further, we have used the GW dispersion relation to make a first order estimation of the horizontal scale sizes. This method of deriving neutral GW characteristics through radio measurement technique is effective for the daytime conditions and opens up new possibilities of investigations of the wave dynamical behaviour in the upper atmosphere during all weather conditions.

Description: Abstract This study examines the response of the nighttime Thermosphere‐Ionosphere System (TIS) over a dip equatorial station, Trivandrum (8.5o N, 77o E, 0.5o dip lat.) to Prompt Penetration Electric Field (PPEF) events occurred on 05 January 2016 and 06 March 2016. The investigation is based on nightglow emission measurements at wavelengths OI 777.4 nm, OI 630.0 nm and OI 557.7 nm using a multi‐wavelength nighttime photometer. These emissions emanate from different altitude regions of the TIS, i.e. 557.7 nm from ~100 km, 630.0 nm from ~220 km and 777.4 nm from ~350 km respectively. It has been observed that, during the westward PPEF event, the intensities of 777.4 nm and 630.0 nm nightglow emissions enhance whereas during the eastward PPEF event the intensities decrease. Similarly, though it is not very prominent, 557.7 nm emission also exhibits a small enhancement/decrease during the westward/eastward PPEF events. The ionospheric base height obtained from a collocated Digital Ionosonde shows that during the eastward (westward) PPEF event the ionospheric layer moves upward (downward). The downward (upward) layer movement increases (decreases) the plasma density in the centroid of these airglow emissions, which in turn enhances (decreases) the emission rates. The study demonstrates the coupling between interplanetary medium and neutral TIS during nighttime PPEF events.

Description: Abstract The most significant error of the radio occultation (RO) ionospheric retrieval is brought by the spherical symmetric assumption of electron density in Abel inversion. We developed an improved inversion method that attaches additional horizontal constraints based on a background model in Guo et al. (2015, https://doi.org/10.1016/j.jastp.2014.12.008), and found the effect of the method depends greatly on the accuracy of the background model. In this study an improved ionospheric model, which applies RO‐based F2‐layer parameters and an adaptive topside model into the International Reference Ionosphere (IRI), is employed as a new choice of background model. The improved inversion is evaluated in several aspects in the mid‐ and low‐latitudinal regions during equinox season (from March 19 to April 18 in 2008 and 2012) and throughout the year. Results show that the artificial plasma caves underneath the equatorial ionization anomaly (EIA) crests caused by Abel inversion is to some extent eliminated during the daytime, and the negative electron densities in the E layer are also reduced by the improved method. The comparison between COSMIC events and co‐located incoherent scatter radar (ISR)/ionosonde observations indicates comparable accuracy of both methods. Moreover, the Global Ionosphere Maps (GIM) is introduced as true reference of the vertical total electron density content (VTEC). The mean VTEC errors derived through the improved inversion method are less than that of Abel inversion in the global scope.

Description: Abstract This study investigates the role of stratospheric Quasi‐Biennial Oscillation (QBO) in modulating the response of equatorial/low latitude ionosphere over the Indian sector to the major Sudden Stratospheric Warmings (SSWs) occurred during the period 2003 to 2013. The analysis based on the Equatorial Electrojet (EEJ)‐induced surface magnetic field, Total Electron Content (TEC), and mesospheric wind data reveals that the equatorial ionosphere responds in distinctly different ways to the SSWs occurred during different QBO phases. The peaking time of the EEJ and occurrence time of Counter Electrojet (CEJ) displays a shift towards morning/evening sector during the westward/eastward phase of the QBO as inferred from the zonal mean zonal wind at 10 hPa level (~30 km). The analysis reveals that the enhancement and depletions seen in TEC over both the equatorial and low‐latitude ionosphere display a shift towards morning/evening during the westward/eastward phase of the QBO. The upper mesospheric tides (diurnal and semidiurnal) estimated using the meteor radar measured winds over the dip equator also exhibit similar shift in their phases. The occurrence of periodic CEJs during the SSW period is found to be coinciding with enhancement in polar stratospheric temperature and in close association with the increased amplitude of the semidiurnal tide. These observations clearly vindicate that the phase of the QBO plays a crucial role in structuring the equatorial electrodynamics and electron density distribution over the low‐latitudes during the SSW events.

Description: Abstract Photos of a spectacular optical phenomenon, nicknamed “STEVE” show finely structured, purple colored, east‐west arcs spanning the sky. These purple Subauroral Arc Emissions (SAE‐s) are associated with Sub Auroral Ion Drifts, often accompanied by separate green arcs frequently displaying magnetic field aligned rays suggesting charge particle excitation. Both types of these arcs and polar auroras appear in some photos. Splitting the images into red, green and blue (RGB) channels allowed comparison of color ratios of the three phenomena. Wavelength calibration of the camera verified that the dominant atmospheric auroral emissions, 630.0 nm O(1D), the O(1S) 557.7 nm and N2+1N bands, were cleanly separated in the RGB channels of the camera. In the absence of a spectrogram the ratios between the color channels were interpreted in terms of possible excitation mechanisms. The purple arcs contained an excess of blue, presumably N2+1N intensity. This excess production could be due to the excitation of N2+ ions that were ionized through charge exchange with O+. The green companion arcs appear to be purely green (557.7) with almost no blue and minimal red suggesting excitation by low energy electrons excitation at altitudes 〉100 and 〈150km.

Description: Abstract One of the most popular long‐term datasets of energetic particles used in, e.g., long‐term radiation belt studies and in atmospheric/climate studies is perhaps the NOAA/POES (Polar Orbiting Environmental Satellites) dataset, which extends nearly continuously from 1979 to present. The present study aims to construct a new homogeneous long‐term composite record of daily latitude distributions of energetic electrons based on the MEPED (Medium Energy Proton and Electron Detector) data. Part 1 of this study corrected the data for temporally varying background noise related to cosmic rays and for the drift in the orientation of satellite orbital planes. The present paper addresses the final and most severe problem for the data homogeneity, caused by the difference of telescope pointing directions in older SEM‐1 and newer SEM‐2 versions of the MEPED instrument. Because the telescope pitch angles and the electron pitch angle distribution change with latitude the difference in SEM‐1 and SEM‐2 fluxes depends on latitude and varies from time to time. The systematic flux differences between SEM‐1 and SEM‐2 can range between a factor of 1.5 to more than an order of magnitude. Novel statistical methodology based on principal components and canonical correlation mapping is presented here to robustly transform the daily SEM‐1 electron latitude distributions into SEM‐2 level. The data from different POES satellites is then combined into a spatially and temporally homogeneous composite series, which is well suited, e.g., for long‐term studies of radiation belts and precipitation related atmospheric ionization and its chemical and dynamical effects in the atmosphere/climate system.

Description: Abstract In magnetized plasmas such as the ionosphere, electric currents develop in regions of strong density gradients to balance the resulting plasma pressure gradients. These currents, usually known as diamagnetic currents decrease the magnetic pressure where the plasma pressure increases, and vice versa. In the low latitude ionosphere, equatorial plasma depletions (EPDs) are well‐known for their steep plasma density gradients and adverse effect on radio wave propagation. In this paper, we use continuous measurements of the magnetic field and electron density from the ESA's Swarm constellation mission to assess the balance between plasma and magnetic pressure across large‐scale EPDs. The analysis is based on the magnetic fluctuations related to diamagnetic currents flowing at the edges of EPDs. This study shows that most of the EPDs detected by Swarm present a decrease of the plasma pressure relative to the ambient plasma. However, EPDs with high plasma pressure are also identified mainly in the vicinity of the South Atlantic magnetic anomaly. From the electron density measurements, we deduce that such an increase in plasma pressure within EPDs might be possible by temperatures inside the EPD as high as twice the temperature of the ambient plasma. Due to the distinct location of the high‐pressure EPDs, we suggest that a possible heating mechanism might be due to precipitation of particle from the radiation belts. This finding corresponds to the first observational evidence of plasma pressure enhancements in regions of depleted plasma density in the ionosphere.

Description: Abstract We present a numerical algorithm to identify ion diffusion regions (IDRs) in the geomagnetic tail, and test its applicability. We use 5 criteria applied in three stages. (i) Correlated reversals (within 90 s) of Vx and Bz (at least 2 nT about zero; GSM coordinates); (ii) Detection of Hall electric and magnetic field signatures; and (iii) strong (≥ 10 mV/m) electric fields. While no criterion alone is necessary and sufficient, the approach does provide a robust, if conservative, list of IDRs. We use data from the Magnetospheric Multiscale Mission (MMS) spacecraft during a 5‐month period (May 1 to September 30, 2017) of near‐tail orbits. We find 148 events satisfying step 1, 37 satisfying steps 1 and 2, and 17 satisfying all three, of which 12 are confirmed as IDRs. All IDRs were within the X‐range [‐24, ‐15] RE and the majority occurred during traversals of a tailward‐moving X‐line. 11 of 12 IDRs were on the dusk‐side despite approximately equal residence time in the plasma sheet (56.5% dusk vs 43.5% dawn). MMS could identify signatures of 4 quadrants of the Hall B‐structure in 3 events and 3 quadrants in 7 events. The events we report commonly display Vx reversals greater than 400 km/s in magnitude, normal magnetic field reversals often 〉10 nT in magnitude, maximum which are often well in excess of the threshold for stage 3. Our results are then compared with the set of IDRs identified by visual examination from Cluster in the years 2000‐2005.

Description: Abstract The magnetosheath and near‐Earth solar wind emit X‐rays due to charge‐exchange between the extended atmosphere and highly ionized particles in the solar wind. These emissions can be used to remotely sense the dynamic processes in this region. The Solar Wind Magnetosphere Ionosphere Link Explorer mission will carry out these measurements. In a previous paper, we looked at the effect of photon counting statistics on determining the location of the magnetopause and bow shock. In this paper we explore, through simulations, the more challenging question of orbital viewing geometry bias when the model and the emissions do not match each other exactly. Our simulations conclude that while care must be taken to avoid false minima in the fitting, there is very little to no orbital bias in extracting the position and large‐scale shape of the magnetopause and bow shocks from 2‐D X‐ray images from the future Solar Wind Magnetosphere Ionosphere Link Explorer mission.

Description: Abstract During geomagnetic storms, the magnetic field in the outer belt is known to be significantly distorted by the solar wind, such that the outer belt dynamics can be greatly impacted by the magnetic field topology. In this context, we develop a reduced Fokker‐Planck code that includes the effects related to realistic magnetic field models (by using the existing dedicated LANLGeoMag library) through the steps of pre‐processing, post‐processing, and, for the first time, during the computation of the reduced Fokker‐Planck diffusion equation itself. We perform the solutions of the reduced Fokker‐Planck equation in the framework of the geomagnetic storm that occurred from October 9 to October 15, 1990. With the use of CRRES observations, the magnetic field model is shown to strongly affect the way of conciliating theory with observations (processing steps). More specifically, we explain analytically and numerically why the use of a dipole field can lead to misleading interpretations on the local enhancements (attributed to local acceleration) displayed by the electron distribution function, resulting in inaccurate simulations results at large L‐shells. The consideration of a realistic field does not produce any artificial peaks. With such corrected data sets, a great part of the dynamics can be described by radial transport and is thus better reproduced by the simulations. This crucial importance of the field geometry is further emphasized with the calculation of unidirectional, omnidirectional, and integral electron fluxes and their accuracy is quantified thanks to dedicated metrics.

Description: Abstract The wave normal angle of excited whistler waves was previously considered to be controlled by the parallel plasma beta (β∥h) of anisotropic hot electrons, while the effects of thermal electrons were usually neglected. By combining both the linear theoretical and 2‐D PIC simulation models, we have investigated the effects of thermal electrons on the whistler anisotropy instability. In the high‐beta (β∥h ≥ 0.025) regime, the wave normal angle of the dominant whistler mode with the largest growth rate is always zero degree, which is not affected by thermal electrons. While, its wave frequency and linear growth rate decrease with the density and temperature of thermal electrons. These results are also confirmed by PIC simulations. In the low‐beta (β∥h ≤ 0.025) regime, with the increase of the density and temperature of thermal electrons, the wave normal angle of the dominant whistler mode turns to zero from a large value. This change could be due to the stronger damping caused by thermal electrons for oblique whistler mode, since oblique wave usually has a smaller cyclotron resonant velocity than parallel wave. PIC simulations also show a consistent result, but reproduce a broad magnetic spectrum, even in the case including sufficient thermal electrons. Furthermore, thermal electrons with large parallel velocities are resonantly accelerated in the perpendicular direction, while part of hot electrons are trapped and accelerated in the parallel direction. Our study suggests that the wave normal angle of whistler mode in the Earth's magnetosphere could be determined by both anisotropic and thermal electrons.

Description: Abstract The persistent two‐peaked vertical structure of the Martian ionosphere is created by extreme and far ultraviolet radiation whose energies respectively determine their ionization altitude. A third low‐altitude transient layer (previously referred to as M3 or Mm) has been observed by radio occultation techniques and attributed to meteor ablation. However, recent remote sensing and in‐situ observations disfavor a meteoric origin. Here we propose an alternative hypothesis for these apparent layers associated with impact ionization from penetrating solar wind ions, previously observed as proton aurora. Localized ionization, occurring non‐globally at a given altitude range, breaks the symmetry assumed by the radio occultation technique, and creates electron layers apparently lower in the ionosphere than their true altitude. This may occur when the upstream bowshock is altered by a radial interplanetary magnetic field configuration, which allows the solar wind to penetrate directly into the thermosphere. This localized ionization hypothesis provides an explanation for apparent layers’ wide variation in heights and their transient behavior. Moreover, this hypothesis is testable with new observations by the Mars Atmospheric and Volatile EvolutioN (MAVEN) Radio Occultation Science Experiment (ROSE) in future Mars years. This hypothesis has implications for the ionospheres of Venus and Titan, where similar transient layers have been observed.

Description: Abstract We analyze quiet‐time data from the Gravity Field and Ocean Circulation Explorer (GOCE) satellite as it overpassed the Southern Andes at z ≃ 275km on 5 July 2010 at 23 UT. We extract the 20 largest traveling atmospheric disturbances (TADs) from the density perturbations and cross‐track winds using Fourier analysis. Using gravity wave (GW) dissipative theory that includes realistic molecular viscosity, we search parameter space to determine which hotspot TADs are GWs. This results in the identification of 17 GWs having horizontal wavelengths λH = 170 − 1850km, intrinsic periods τIr = 11 − 54min, intrinsic horizontal phase speeds cIH = 245 − 630m/s, and density perturbations .We unambiguously determine the propagation direction for 11 of these GWs, and find that most had large meridional components to their propagation directions. Using reverse ray‐tracing, we find that 10 of these GWs must have been created in the mesosphere or thermosphere. We show that mountain waves (MWs) were observed in the stratosphere earlier that day, and that these MWs saturated at z ∼ 70 − 75 km from convective instability. We suggest that these 10 GOCE hotspot GWs are likely tertiary (or higher‐order) GWs created from the dissipation of secondary GWs excited from the body forces created from MW breaking. We suggest that the other GW is likely a secondary or tertiary (or higher‐order) GW. This study strongly suggests that the hotspot GWs over the Southern Andes in the quiet‐time middle winter thermosphere can not be successfully modeled by conventional global circulation models where GWs are parameterized and launched in the troposphere.

Description: Abstract MESSENGER measurements taken during passes over Mercury's dayside hemisphere indicate that on 4 occasions the spacecraft remained in the magnetosheath even though it reached altitudes below 300 km. During these disappearing dayside magnetosphere (DDM) events, MESSENGER did not encounter the magnetopause until it was at very high magnetic latitudes, ~ 66 to 80o. These DDM events stand‐out with respect to their extremely high solar wind dynamic pressures, Psw ~ 140 to 290 nPa, and intense southward magnetic fields, Bz ~ ‐ 100 to ‐ 400 nT, measured in the magnetosheath. In addition, the bow shock was observed very close to the surface during these events with a subsolar altitude of ~ 1200 km. It is suggested that DDM events, which are closely associated with coronal mass ejections, are due to solar wind compression and/or reconnection‐driven erosion of the dayside magnetosphere. The very low altitude of the bow shock during these events strongly suggests that the solar wind impacts much of Mercury's sunlit hemisphere during these events. More study of these disappearing dayside events is required, but it is likely that solar wind sputtering of neutrals from the surface into the exosphere maximizes during these intervals.

Description: Abstract Signals from VLF transmitters can leak from the Earth‐ionosphere wave guide into the inner magnetosphere, where they propagate in the whistler mode and contribute to electron dynamics in the inner radiation belt and slot region. Observations show that the waves from each VLF transmitter are highly localised, peaking on the nightside in the vicinity of the transmitter. In this study we use ~5 years of Van Allen probe observations to construct global statistical models of the bounce‐averaged pitch angle diffusion coefficients for each individual VLF transmitter, as a function of L*, Magnetic Local Time (MLT) and geographic longitude. We construct a 1D pitch‐angle diffusion model with implicit longitude and MLT dependence to show that VLF transmitter waves weakly scatter electrons into the drift loss cone. We find that global averages of the wave power, determined by averaging the wave power over MLT and longitude, capture the long‐term dynamics of the loss process, despite the highly localised nature of the waves in space. We use our new model to assess the role of VLF transmitters waves, hiss waves, and Coulomb collisions on electron loss in the inner radiation belt and slot region. At moderate relativistic energies, E~500 keV, waves from VLF transmitters reduce electron lifetimes by an order of magnitude or more, down to the order of 200 days near the outer edge of the inner radiation belt. However, VLF transmitter waves are ineffective at removing multi‐MeV electrons from either the inner radiation belt or slot region.

Description: Abstract The atmosphere of the Jovian satellite Io is constantly being lost to the surrounding magnetosphere of Jupiter. The material is ionized and then distributed by Jupiter's magnetic field into a torus around Jupiter called the Io plasma torus. This plasma affects radio signals as they propagate from the Juno spacecraft to Earth during the spacecraft's perijove passes. During Perijoves 3, 6, and 8 we determine the total electron content in the Io plasma torus using two‐way tracking data from Juno. We find that the location of the torus is displaced from predictions that use the VIP4 offset tilted dipole approximation. The displacements are consistent with those found in ground‐based observations. The peak total electron content and scale height are found for two different regions of the torus, the cold inner torus and a warmer torus beyond 5.5 RJ. Properties of the cold torus vary appreciably with System III longitude, but properties of the torus beyond 5.5 RJ do not.

Description: Abstract Daytime thermospheric winds observed by the balloon‐borne instrument HIWIND (High altitude Interferometer WIND experiment) during two flights in June 2011 and 2018 from Kiruna (68°N, 20°E), along with simultaneous EISCAT (European Incoherent SCATter radar) ion drift data, are analyzed. NCAR TIEGCM (Thermosphere Ionosphere Electrodynamics General Circulation Model) simulations for both flights are compared with observations. The observed thermospheric winds from the two flights have many similarities. HIWIND observed thermospheric winds tend to be equatorward during the morning hours before noon and close to zero in the afternoon. In contrast, TIEGCM predicts poleward winds before noon and near zero in the afternoon. Southward IMF Bz, occurring as the balloon passed through morning, was associated with greater equatorward meridional winds. The TIEGCM simulated zonal winds have large differences with observations under more active conditions. The second HIWIND flight confirms some important results from the first flight and further shows thermospheric wind variations under different IMF conditions. HIWIND observations in general provide invaluable data for model validation and highlight deficiencies in current high latitude simulations.

Description: Abstract A frequency‐tunable resonance scattering lidar with high temporal/vertical resolutions (1 min/15 m) observed sporadic calcium ion (Ca+s) layers at ~100 km over Tachikawa (geographical/geomagnetic latitude: 35.7°N/27.1°N), Japan, on 21–22 August 2014. Simultaneously, sporadic E (Es) parameters and medium‐scale traveling ionospheric disturbances (MSTIDs) were observed by an ionosonde and Global Navigation Satellite System receiver network, GEONET, respectively. The maximum densities of the Ca+ and electrons in the Es layer had a strong positive correlation. As observation started ~23:30 LT, the Ca+s layer and the associated Es layer descended at ~2.8 km/hr with density irregularities including Kelvin‐Helmholtz billow‐like structures suggesting the presence of background neutral wind shear and instability. And the total electron content variations showed large amplitude associated with the MSTIDs at an altitude of 300 km in synchronization with the Ca+ column abundance surges at 100 km over Tachikawa; in their respective E and F region locations connected by geomagnetic field line these irregularities are found to vary in phase. At ~02:00 LT, the Ca+s layer stopped descending at ~100 km due to larger ion‐neutral collision frequency in the lower altitudes and resided there quietly until sunrise; both Ca+ column abundance enhancements and the large total electron content variation disappeared as the descent of the Ca+s layer stopped, implicating that the MSTID structure cannot be sustained without the density irregularities of the Es layer. This is the first synchronous observation of the coupling between the Es density irregularities and the MSTIDs in the F region along a common magnetic flux tube.

Description: Abstract One of the most important sources of magnetospheric plasma is particle entry through the distant magnetotail boundary, the nightside magnetopause. This entry mechanism depends on the magnetopause configuration. Off the equator, the strong lobe magnetic field renders the magnetopause a tangential or a rotational discontinuity, and thus the magnetosheath field orientation predominantly controls particle entry through magnetic reconnection. At the equatorial, distant tail magnetopause, however, the magnetic field's control of particle entry is diminished because the plasma beta there is large on both sides of the boundary. Thus, transport there can be significantly different from that at the dayside and off‐equatorial magnetopauses. Using observations from two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes, we investigate plasma transport mechanisms around the distant equatorial magnetopause. We find that transport occurs as a series of abrupt transitions in density, ion and electron temperatures, and ion kinetic energy of spatial scales as small as a typical plasma sheet ion gyroradius. Analysis of the particle phase space density reveals that an energy‐selection mechanism controls electron transport across magnetopause, whereas ion transport is likely controlled by spatial diffusion driven by low‐frequency magnetic field fluctuations. We discuss the importance of these fluctuations for the magnetopause structure (e.g., the thickness of the transitions in plasma density, ion and electron temperatures, and ion kinetic energy).

Description: Abstract An analysis of the entire dataset of 5600 electron density profiles returned by Mars Global Surveyor's Radio Science Experiment is carried out to study the physical characteristics of Martian low‐altitude plasma layer (M layer). Our analysis suggests that this layer is predominantly observed during low and moderate solar activity periods, in northern autumn. The critical ionospheric parameters (electron density and height) of this M layer are found not to show a definitive correlation with solar zenith angle. In contrast to earlier reports where meteoroid ablation was proposed to cause Total Electron Content (TEC) enhancements, we report that the maximum contribution from this layer (TECM) is only about 5.5%, while the contribution is 3.7% during meteor shower, suggesting M layer occurrence does not depend upon meteor shower nor on dust storm. It is observed that the M layer occurrence increases as the Martian E region becomes prominent and well‐defined, suggesting that the source which causes M layer possibly leads to more pronounced E layers. Southern hemisphere profiles were found to behave differently from northern hemisphere profiles, possibly due to crustal magnetic fields. Large surges observed in Martian F1 layer peak height during consecutive occultations (~2 hours apart) are found not to show any correlation with the occurrence of M layer and are not influenced by dust storms.

Description: Abstract We use hourly mean magnetic field measurements from 34 mid latitude geomagnetic observatories between 1900 and 2015 to investigate the long‐term evolution and driving mechanism of the large‐scale external magnetic field at ground. The Hourly Magnetospheric Currents (HMC) index is derived as a refinement of the annual version AMC (Pick & Korte, 2017, https://doi.org/10.1093/gji/ggx367). HMC requires an extensive revision of the observatory hourly means. It depends on three third party geomagnetic field models used to eliminate the core, the crustal and the ionospheric solar‐quiet (Sq) field contributions. We mitigate the dependency of HMC on the core field model by subtracting only non‐dipolar components of the model from the data. The separation of the residual (dipolar) signal into internal and external (HMC) parts is the main methodological challenge. Observatory crustal biases are updated with respect to AMC and the Sq field estimation is extended to the past based on reconstructed solar radio flux (F10.7). We find that HMC has more power at low frequencies (periods ≥ one year) than the Dcx index, especially at periods relevant to the solar cycle. Most of the slow variations in HMC can be explained by the open solar magnetic flux (OSF). There is a weakly decreasing linear trend in absolute HMC from 1900 to present, which depends sensitively on the data rejection criteria at early years. HMC is well suited for studying long‐term variations of the geomagnetic field.

Description: Abstract We develop a new empirical model of Jupiter's equatorial current sheet, or magnetodisk, constructed by combining successful elements from several previous models. The new model employs a disk‐like current of constant north‐south thickness in which the current density is piecewise dependent on the distance ρ from Jupiter's dipole axis, proportional to ρ‐1 at distances between ~7 and ~30 RJ and again at distances between ~50 and ~95 RJ, and to be continuous in value but proportional to ρ‐2 at distances between. For this reason we term the model the Piecewise Current Disk (PCD) model. The model also takes into account the curvature of the magnetodisk with distance and azimuth due to finite radial propagation speed and solar wind effects. It is taken to be applicable in the radial distance range between ~5 and ~60 RJ. Optimized parameters have been determined for Juno magnetic field data obtained on Perijove‐01, with the model showing overall the lowest RMS deviation from the data compared with similarly optimized earlier models.

Description: ABSTRACT The present study investigates the dependence of the local auroral electrojet (AEJ) intensity on solar illumination by statistically examining northward geomagnetic disturbances in the auroral zone in terms of the solar zenith angle (SZA) χ. It is found that on the dayside, both westward and eastward electrojets (WEJ and EEJ) are more intense for smaller χ suggesting that the solar EUV‐induced conductance is the dominant factor for the AEJ intensity. On the nightside, in contrast, the χ dependence of the AEJ intensity, if sorted solely by the magnetic local time (MLT), apparently depends on the station longitude and hemisphere. However, if additionally sorted by the dipole tilt angle (DTA) ψ, a consistent pattern emerges. That is, although χ and ψ are correlated, the SZA and DTA have physically different effects on the AEJ intensity. The nightside AEJ, especially the WEJ, tends to be more intense for smaller|ψ|. Moreover, whereas the WEJ is statistically more intense when the ionosphere is dark, the EEJ is more intense when it is sunlit. The preference of the WEJ for the dark ionosphere prevails widely in MLT from premidnight to dawn, and therefore, it cannot be attributed to the previously proposed processes of the preferred monoenergetic or broadband auroral precipitation in the dark ionosphere. Instead, it may be explained, at least morphologically, in terms of the conductance enhancement due to the diffuse auroral precipitation, which is also prevalent from premidnight to dawn and is more intense in the dark hemisphere.

Description: Abstract The geomagnetic storm‐driven ionospheric changes and the involved processes are interesting and challenging topics in understanding and predicting the ionosphere. In this study we investigate the response of the ionosphere to geomagnetic disturbances during the 30‐day incoherent scatter radar measurements conducted at Millstone Hill (42.6°N, 71.5°W) from 4 October to 4 November 2002. During geomagnetically disturbed periods, while the peak electron density of the F2 layer (NmF2) and total electron content deviate remarkably from the quiet time ones in a similar way, the incoherent scatter radar measurements reveal that the changes in electron density are frequently different between low and high altitudes. The electron density is significantly depleted at low altitudes; however, at topside it either changes slightly or sometime is enhanced. The enhanced vertical scale height around 600 km under geomagnetically active conditions implies that the topside electron density profiles become much steeper. The increase in the peak height of F2 layer (hmF2) indicates the upward motions under the action of the storm‐driven dynamic processes. Further, sometimes strong differences are shown in total electron content between Millstone Hill and longitude 100°W. The competing contributions from dynamic processes and disturbance composition to the storm‐time ionospheric changes over Millstone Hill are indicated in the different responses in electron density at the bottomside and topside of the ionosphere.

Description: Abstract The total electron content (TEC) data from Global Ionosphere Maps (GIM) are used to obtain the tidal modes through two‐dimensional Fourier transform in both UT (universal time) and LT (local time) frames. In the LT frame, a north‐south TEC asymmetry is observed along the longitude, where there is a large displacement of the geomagnetic equator from the geographic equator. The phases of tidal modes lead to a constructive or destructive interference of contributing tidal modes, producing different zonal waves and longitudinal peaked structures at different local time (tLT). The summation of all non‐migrating zonal waves in the low latitude region (geomagnetic latitude 〈 30°) produces a peaked structure with two to four major peaks at most local hours. The amplitude of this peaked structure in June solstice is smaller than those in other seasons. During solar maximum (minimum), the amplitude of peaked structure has a value ~20 (~12) TECu with 2 to 4 (1 to 3) peaks. The locations of peaked structures formed by the non‐migrating tides in 2006 (solar minimum) and in 2014 (solar maximum) are similar. The Weddell Sea Anomaly (WSA) is found to have a density enhancement in the region from 135°E to 90°W and from 40°S to 80°S with an amplitude ~19.85 (~5.07) TECu during March equinox (December solstice) of solar maximum (minimum).

Description: Abstract Twenty high‐inclination ring‐grazing orbits occurred in the final period of the Cassini mission. These orbits intercepted a region of intense Z‐mode and narrowband (NB) emission [Ye et al., 2010] along with isolated, intense upper hybrid resonance (UHR) emissions that are often associated with NB source regions. We have singled out such UHR emission seen on earlier Cassini orbits that also lie near the region crossed by the ring‐grazing orbits. These previous orbits are important because Cassini electron phase‐space distributions are available and dispersion analysis can be performed to better understand the free energy source and instability of the UHR emission. We present an example of UHR emission on a previous orbit that is similar to that observed during the ring‐grazing orbits. Analysis of the observed plasma distribution of the previous orbit leads us to conclude that episodes of UHR emission and NB radiation observed during the ring‐grazing orbits are likely due to plasma distributions containing loss cones, temperature anisotropies, and strong density gradients near the ring plane. Z‐mode emissions associated with UHR and NB emission can be in Landau resonance with electrons to produce scattering or acceleration [Woodfield et al., 2018].

Description: Abstract Relativistic electron microbursts are an important electron loss process from the radiation belts into the atmosphere. These precipitation events have been shown to significantly impact the radiation belt fluxes and atmospheric chemistry. In this study we address a lack of knowledge about the relativistic microburst intensity using measurements of 21,746 microbursts from the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX). We find that the relativistic microburst intensity increases as we move inward in L, with a higher proportion of low‐intensity microbursts (〈2,250 [MeV cm2 sr s]−1) in the 03–11 magnetic local time region. The mean microburst intensity increases by a factor of 1.7 as the geomagnetic activity level increases and the proportion of high‐intensity relativistic microbursts (〉2,250 [MeV cm2 sr s]−1) in the 03–11 magnetic local time region increases as geomagnetic activity increases, consistent with changes in the whistler mode chorus wave activity. Comparisons between relativistic microburst properties and trapped fluxes suggest that the microburst intensities are not limited by the trapped flux present alongside the scattering processes. However, microburst activity appears to correspond to the changing trapped flux; more microbursts occur when the trapped fluxes are enhancing, suggesting that microbursts are linked to processes causing the increased trapped fluxes. Finally, modeling of the impact of a published microburst spectra on a flux tube shows that microbursts are capable of depleting 〈500‐keV electrons within 1 hr and depleting higher‐energy electrons in 1–23 hr.

Description: Abstract Instances of multiple excitation of large‐scale gravity waves in the auroral region by solar wind Alfvénic fluctuations are rare. In this paper we report an unusual instance, in which sequential large‐scale gravity waves launched from northern and southern auroral regions are clearly linked to intermittent southward magnetic fields of Alfvénic fluctuations embedded in three successive stream interaction regions. The continuous gravity wave activity persists for ∼7 days, the longest duration currently tracked. The Alfvénic fluctuations are mainly dominated by Alfvén and slow waves, which might play a role in the solar wind‐magnetosphere interaction, leading to impulsive auroral electrojet activity and in turn exciting the sequential gravity waves.

Description: Abstract We analyze a recent geomagnetic storm event on 7‐8 September 2017 to investigate the impact of geomagnetic storm on the Precise Orbit Determination (POD) of Swarm constellation. The storm time performance of POD is analyzed. The quality of Swarm orbits are severely degraded during the storm main phase on September 8 and the maximum precision degradation reached over 10 cm. The enhanced thermospheric mass density at Swarm altitude during the storm enlarges the atmosphere drag for low Earth orbit (LEO) satellites, which makes main contributions to the storm time degradation of Swarm orbit. This negative effect of enhanced atmosphere drag on the orbit estimation is mostly suppressed by estimating a more frequent atmosphere drag parameter. The higher‐order ionospheric effects on the POD of Swarm are also analyzed. The Vertical Total Electron Content (VTEC) derived from the Swarm onboard GPS receiver presents a larger enhancement on the dayside at low and mid‐latitudes during the storm main phase. This leads to an increase in the high‐order ionospheric effects, especially in the second‐order terms. No evident precision improvement is observed after correcting the high‐order ionospheric effects. The results demonstrate that during this geomagnetic storm, the enhanced thermospheric neutral density serves as a stronger error source than the enhanced ionospheric plasma density for the LEO satellite orbit determination processing.

Description: Abstract On June 21–22, 2015, three consecutive interplanetary shocks slammed into the Earth's magnetosphere. Immediately after the third shock at 18:36 UT on June 22, marked by an exceptional sudden storm commencement with an amplitude of ΔSYM‐H = ∼106 nT, a major geomagnetic storm commenced. In the present study, a multi‐instrument approach comprising of observations, data analysis, and modeling is used to examine the global ionospheric response. Results show that enhanced storm‐time processes produced major total electron content (TEC) variations at different latitudes, longitudes, and phases of the storm. A closer inspection of the TEC observations reveals strong longitudinal and hemispherical asymmetry. In addition, multiple equatorward and poleward propagating traveling ionospheric disturbances (TIDs) were detected in the TEC data. Equatorward propagating TIDs are consistent with vertical neutral winds simulated from TIE‐GCM, however, poleward TIDs were not reproduced in the model. We find that a combination of driving processes including enhanced high‐latitude injection, prompt penetration electric fields, disturbance dynamo effect, neutral winds, and composition changes were acting at different stages of the storm.

Description: Abstract We present a statistical analysis with 100% duty cycle and non‐time‐averaged amplitudes of the prevalence and distribution of high‐amplitude 〉 50 pT whistler‐mode waves in the outer radiation belt using 5 years of Van Allen Probes data. Whistler‐mode waves with high magnetic field amplitudes are most common above L=4.5 and between MLT of 0‐14 where they are present approximately 1‐6% of the time. During high geomagnetic activity, high‐amplitude whistler‐mode wave occurrence rises above 25% in some regions. The day‐side population are more common during quiet or moderate geomagnetic activity and occur primarily 〉 5 degrees from the magnetic equator, while the night‐to‐dawn population are enhanced during active times and are primarily within 5 degrees of the magnetic equator. These results are different from the distribution of electric field peaks discussed in our previous paper covering the same time period and spatial range. Our previous study found large‐amplitude electric field peaks were common down to L=3.5 and were largely absent from afternoon and near‐noon. The different distribution of large electric and magnetic field amplitudes implies that the low‐L component of whistler‐mode waves observed previously are primarily highly oblique, while the dayside and high‐L populations are primarily field‐aligned. These results have important implications for modeling radiation belt particle interactions with chorus, as large‐amplitude waves interact non‐linearly with electrons, resulting in rapid energization, de‐energization, or pitch‐angle scattering. This also may provide clues regarding the mechanisms which can cause significant whistler‐mode wave growth up to more than 100x the average wave amplitude.

Description: Abstract Suprathermal electron depletions are structures of the nightside ionosphere of Mars resulting from an equilibrium between electron loss and creation processes. Photoionization of oxygen and carbon dioxide by UV and EUV photons is the main ionization process of the Martian atmosphere. The observation of suprathermal electron depletions is strongly unexpected in the portion of the Martian environment where photoionization can occur. This region is delimited by the UltraViolet (UV) terminator, behind which no UV ionizing photons are detected. In this study suprathermal electron depletions are used to determine the position of the UV terminator thanks to MAVEN observations. The MAVEN spacecraft is now in its fourth year of data recording and has already covered more than one Martian year, a large range of latitude, local time and solar zenith angle in the nightside down to 110 km altitude. This coverage enables us to determine the approximate position of the UV terminator over one Martian year. We then investigate the variation of its position on the dawn and dusk sides and depending on seasons. Our results are compared with models of the Martian atmosphere and in‐situ data of the atmospheric composition which all highlight an asymmetry between the dusk and the dawn sides at equinox. However, models show an inversion in the position of the dusk and the dawn UV terminator at perihelion and aphelion, which cannot yet be confirmed or disproved by the data.

Description: Abstract We studied the spatiotemporal structure of ground magnetic pulsations on the dayside by displaying magnetic field perturbations detected by the European quasi‐Meridional Magnetometer Array (EMMA) as 2‐D images in the magnetic L value versus time space, called EMMAgrams. We generated EMMAgrams from observations made on 15 August 2015, including a previously studied pulsation event associated with an interplanetary shock. In addition to signatures of field line resonance (FLR) driven by a cavity mode oscillation, we found poleward‐propagating structures with L‐independent periods in the Pc2 band. The Pc2 structures are attributed to periodic magnetohydrodynamic pulses (upstream waves) originating from the ion foreshock and propagating in the magnetosphere along the path proposed by Tamao. Ringing of local field lines at L‐dependent periods (transient pulsations) is also clearly detected as dispersive poleward‐propagating structures not only immediately after the shock impact but also during time periods of less obvious external disturbances. A transient pulsation decays after a few wave periods, and cross‐spectral analysis of transient pulsations detected at two stations with a small latitudinal separation indicates elevation of the cross phase in a band delimited by the FLR frequencies at the stations. Successive excitation of transient pulsations by variations of the solar wind dynamic pressure appears to contribute significantly to formation of similar cross‐phase peaks that are widely used in magnetoseismic studies.

Description: Abstract Pluto is embedded in a distinct population of energetic heliospheric ions consisting of interstellar pickup ions with energies of a few keV and suprathermal ions with tens of keV and above. We measure this population using the PEPSSI instrument (Pluto Energetic Particle Spectrometer Science Investigation) onboard the New Horizons spacecraft that flew by Pluto in 2015. Even though the measured ions have gyroradii larger than the size of Pluto and the cross section of its magnetosphere, we find that the boundary of the magnetosphere is depleting the energetic ion intensities by about an order of magnitude close to Pluto. The intensity is increasing exponentially with distance to Pluto and reaches nominal levels of the interplanetary medium at about 190RP distance. Inside the wake of Pluto, we observe oscillations of the ion intensities with a periodicity of about 0.2h. We show that these can be quantitatively explained by the electric field of an ultra low frequency wave and discuss possible physical drivers for such a field. We find no evidence for the presence of plutogenic ions in the considered energy range.

Description: Abstract We apply singular spectrum analysis in order to identify trends and quasi‐periodic oscillations in the aa and Dst series of geomagnetic activity. We also analyze the sunspot number International SunSpot Number (ISSN) and the number of polar faculae Polar Faculae (PF). Singular spectrum analysis provides the eigenvalues and therefore trends and oscillatory components of the four series. ISSN is dominated by a trend (the Gleissberg cycle), followed by 10.6, 35.5 years, two ~8‐year components, 21.4 and 5.3 year. aa shows the same trend, a ~47‐year component, then 10.8, 32.3, 21.8, and a series of three close components at 10.6, 12.2, and 9.2 years, followed by a 6 month seasonal component. PF is dominated by the 20.7‐year period, followed by 10.2, 8.3, 41, and 31 years, then a 5.2 year component. Dst is dominated by a trend, then a strong 6‐month component; next are found a 47‐year component, the 10.6 years and a second seasonal line at 1 year. The ~22‐, ~11‐, and ~5.5‐year components are common to the four indices. These “pseudo harmonic” components are evidence of solar activity. Singular spectrum analysis identifies components that vary in frequency and amplitude. The phase relationships of any two components over time can be studied in detail. An illustration is given by the remarkable phase coherency of the 5.3‐year component. But the components are neither truly periodical nor exact multiples of each other. These differences reflect the complex mechanisms that govern solar‐terrestrial relationships.

Description: Abstract In recent years, the middle atmosphere has evoked great scientific interest as long‐term changes can be clearly captured owing to the large perturbation amplitudes at these altitudes. In the present study, more than 25 years of the data are used to investigate the long‐term trends in the middle atmosphere by suitably combining the observations from different techniques (Rocketsonde, High‐Resolution Doppler Imager (HRDI)/Upper Atmosphere Research Satellite (UARS), Halogen Occultation Experiment (HALOE)/UARS, Sounding of the Atmosphere using Broadband Emission Radiometry (SABER)/Thermosphere Ionosphere Mesosphere Energetic Dynamics (TIMED), and Mesosphere‐stratosphere‐troposphere (MST) radar) from Indian region. As different instruments/techniques are used and the time periods are not the same, extreme care has been taken while merging various data sets to obtain meaningful long‐term trends. To understand the observed long‐term trends, Whole Atmosphere Community Climate Model–eXtended model simulations for the Indian low‐latitude regions are also used. A significant cooling trend of ~−1.7 ± 0.5 K/decade between 30 and 80 km is noticed. Large decreasing trend (~5 m/s/decade) in the eastward winds is noticed which is significant between 70 and 80 km only changing from strong eastward wind in 1970s to weak westward wind in recent decade. No significant trends are observed in the meridional wind. These observations are well captured by the Whole Atmosphere Community Climate Model–eXtended model simulations while considering changes in concentrations of greenhouse gases including carbon dioxide (CO2), methane (CH4), water vapor (H2O), and chlorofluorocarbon species that cause depletion of stratospheric ozone (O3). Thus, it is prudent to conclude that long‐term decreasing trends in the zonal winds and cooling trends in the temperature in the middle atmosphere are caused to a large extent by greenhouse gases suggesting the role of anthropogenic changes in the dynamics of the middle atmosphere.

Description: Abstract We present statistical investigation of the high‐latitude ionospheric current systems in the Northern Hemisphere (NH) and Southern Hemisphere (SH) during low (Kp 〈 2) and high (Kp ≥ 2) geomagnetic activity levels. Nearly 4 years of vector magnetic field measurements are analyzed from the two parallel flying Swarm A and C satellites using the spherical elementary current system method. The ionospheric horizontal and field‐aligned currents (FACs) for each auroral oval crossing are calculated. The distributions of the mean values of FACs as well as the horizontal curl‐free and divergence‐free currents in magnetic latitude and magnetic local time for each hemisphere and activity level are presented. To estimate the NH/SH current ratios for the two activity levels, we remove seasonal bias in the number of samples and in the Kp distribution by bootstrap resampling. This is done in such a manner that there are equal number of samples from each season in each Kp bin. We find that for the low activity level, the currents in the NH are stronger than in the SH by 12±4% for FAC, 9±2% for the horizontal curl‐free current, and 8±2% for the horizontal divergence‐free current. During the high activity level, the hemispheric differences are not statistically significant. This suggests that the local ionospheric conditions, such as magnetic field strength or daily variations in insolation, may be important and play a larger role during quiet than disturbed periods. This issue must be studied further.

Description: Abstract In this paper, I sketch a path through my research career in solar and heliospheric physics, recalling some memorable events and discoveries that occurred along the way. This chain of events begins with an influential Time magazine article in 1955, and progresses through a summer at Bell Labs, 9 years at Caltech, 7 years at the Kitt Peak National Observatory, 43 years at the Naval Research Laboratory, and ends with a digitized map of the Sun's Ca II K‐Line emission in 1919 when the AGU was born. Accidents and puzzling results are often the keys to progress and should be examined carefully.

Description: Abstract For the first time, atomic oxygen column content [O]col has been inferred from June daytime monthly median foF1 and foF2 observations at Rome, Juliusruh, Sodankylä, and Boulder to analyze its long‐term variations for the period of ~6 solar cycles. The analysis is interesting in the light of possible anthropogenic impact on the upper atmosphere. After the removal of solar and geomagnetic activity effects from the inferred [O]col variations, the residual linear trends are negative and statistically insignificant at middle latitudes. It is shown that ~93% (the corresponding correlation coefficient is 0.964 ± 0.03) of the whole [O]col variability is explained by solar and geomagnetic activity long‐term variations and only ~7% may be attributed to other processes (reasons) including the anthropogenic impact. Solar and geomagnetic activity contributions to [O]col long‐term variations decrease with time, and this may be related to the low solar activity epoch, which we have entered. The main conclusion is that the long‐term variations of the atomic oxygen column content inferred from ionospheric observations are due to solar and geomagnetic activity; that is, they have a natural origin.

Description: Abstract On 23 February 2014, Van Allen Probes sensors observed quite strong electromagnetic ion cyclotron waves (EMIC) in the outer dayside magnetosphere. The maximum amplitude was more than 14 nT, comparable to 7 % of the magnitude of the ambient magnetic field. The EMIC waves consisted of a series of coherent rising tone emissions. Rising tones are excited sporadically by energetic protons. At the same time, the probes detected drastic fluctuations in fluxes of MeV electrons. It was found that the electron fluxes decreased by more than 30 % during the one minute following the observation of each EMIC rising tone emissions. Furthermore, it is concluded that the flux reduction is a non‐adiabatic (irreversible) process since holes in the particle flux levels appear as drift echoes with energy dispersion. We examine the process of the electron pitch angle scattering by nonlinear wave trapping due to anomalous cyclotron resonance with EMIC rising tone emissions. The energy range of precipitated electrons corresponds to the presumed energy for the threshold amplitude for nonlinear wave trapping. This is the first report of rapid precipitation (〈1 minute) by the mechanism of relativistic electrons by EMIC rising tone emissions and their drift echoes in time observed by spacecraft.

Description: Abstract Characterizing the azimuthal mode number, m, of Ultra Low Frequency (ULF) waves is necessary for calculating radial diffusion of radiation belt electrons. A cross‐spectral technique is applied to the compressional Pc5 ULF waves observed by multiple pairs of GOES satellites to estimate the azimuthal mode structure during the 28‐31 May 2010 storm. We find that allowing for both positive and negative m is important to achieve a more realistic distribution of mode numbers and to resolve wave propagation direction. During the storm commencement when the solar wind dynamic pressure is high, ULF wave power is found to dominate at low mode numbers. An interesting change of sign in m occurred around noon, which is consistent with the driving of ULF waves by solar wind buffeting around noon, creating anti‐sunward wave propagation. The low‐mode ULF waves are also found to have a less global coverage in Magnetic Local Time (MLT) than previously assumed. In contrast, during the storm main phase and early recovery phase when the solar wind dynamic pressure is low and the Auroral Electrojet (AE) index is high, wave power is shown to be distributed over all modes from low to high. The high‐mode waves are found to cover a wider range of MLT than what was previously assumed. Furthermore, to reduce the 2nπ ambiguity in resolving m, a cross‐pair analysis is performed on satellite field measurements for the first time, which is demonstrated to be effective in generating more reliable mode structure of ULF waves during high AE periods.

Description: Abstract In this work, we constructed an ensemble Kalman filter (EnKF) ionosphere and thermosphere data assimilation system using the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) as the background model. We use a sparse matrix method to avoid significant matrix related calculation and storage. A series of observing system simulation experiments (OSSE) have been conducted to assess the performance of the system. The results show that the system optimizes ionosphere drivers efficiently by assimilating electron densities through their covariance. The short‐term forecast capability is enhanced significantly and the effect of initial condition correction lasts for longer than 24 hours. To our knowledge, this is the first study to demonstrate that the EnKF based global ionosphere and thermosphere data assimilation can be conducted without using a supercomputer. This workstation‐based EnKF ionosphere and thermosphere data assimilation system benefits both scientific studies and near real‐time operation.

Description: Abstract The spatial distribution and amplitudes of electron phase space holes observed in the plasma environment near Earth's Moon are presented. When the Moon is in the solar wind, the overwhelming majority of holes in its vicinity occur in its wake, and we attribute them to an instability caused by distortion of the electron distribution by the wake, as predicted by theory. Approximately 30% of these wake holes are statistically correlated with observed magnetic discontinuities nearby, which implies that external effects can influence and trigger these wake instabilities, yet the wake is the determining factor. When the Moon is deep in the Earth's magnetotail, and no detectable lunar wake is present, the hole occurrence is greatly reduced and is distributed approximately homogeneously about the Moon, implicating a different production mechanism. Near the boundary of the magnetotail, homogeneously occurring holes are more frequent, showing that other instabilities associated with the magnetopause region are also active. These results demonstrate ways in which the Earth's Moon is a unique plasma physics laboratory where plasma wake physics and electron instabilities can be studied in detail.

Description: Abstract We study the contribution of the solar wind Poynting flux to the total power input into the magnetosphere. The dominant power delivered by the solar wind is the kinetic energy flux of the particles, which is larger than Ssw by a factor of order , where MA is the Alfvén Mach number. The currents flowing in the bow shock and magnetosheath and the electric field of the solar wind give regions where , which are sources of Poynting flux, generated from the kinetic energy flux. For southward interplanetary magnetic field, is duskward and the currents in the high‐latitude tail magnetopause are also sources of Poynting flux. We show transfer of kinetic energy into the magnetosphere is less efficient than direct entry of by a factor MA. Because MA is typically of order 10, this means that although the power density in the solar wind due to is typically only 1%, it is responsible for of order 10% of the energy input to the magnetosphere. To investigate the effect of this, we add a term to the solar wind‐magnetosphere energy coupling function that allows for and that increases the correlation with the geomagnetic am index for 1995–2017 (inclusive) from 0.908 to 0.924 for 1‐day averages and from 0.978 to 0.979 for annual means. The increase for means on daily or smaller timescales is a small improvement but is significant (at over the 3σ level), whereas the improvement for annual or Carrington rotation means is not significant.

Description: Abstract Lightning has previously been observed to disturb the lower ionosphere. These lightning induced ionospheric perturbations were observed as disturbances on sub‐ionospheric low frequency radio wave propagation with fast rise times 〈3 s and gradual recoveries of duration 〉10 s. Most of these disturbances were observed during night time when ionospheric conditions are most favourable. Here, a day time perturbation in the lower ionosphere was observed using sub‐ionospheric radio remote sensing. The disturbance exhibits a ~60 s rise time with a gradual recovery of 〉200 s. No cloud to ground lightning was coincident with the disturbance onset, however, the intra‐cloud lightning activity of a thunderstorm over the radio receiver was seen to increase at the time of the disturbance. Therefore, the observed disturbance is unlikely to be caused by lightning, yet appears to be associated with the thunderstorm. It is proposed that this disturbance is produced by a pronounced increase in updraught strength that produced a significant change in the quasi‐static electrification of the thunderstorm. This change in electrification is evident in the increase in the intra‐cloud lightning activity of the thunderstorm. The observation provides further evidence of the ionospheric heating effect of thundercloud charge which has implications for lower atmosphere‐ionosphere energy coupling and possibly sprite initiation.

Description: Abstract Using Van Allen Probe EMFISIS wave observations from September 2012 to May 2018, we statistically investigate the distributions of power‐weighted wave normal angle (WNA) of fast Magnetosonic (MS) waves from L= 2‐6 within ±15o geomagnetic latitudes. The spatial distributions show that the MS WNAs are mainly confined within 87o ‐ 89o near the geomagnetic equator and decrease with increasing magnetic latitude. Further quantitative investigation demonstrates that the WNAs normally distribute as a mixture of two Gaussian distributions ranging from 85o to 88o and the tangent of it can decrease as a Kappa distribution function when the waves propagate to higher latitudes. Our study completes the survey of spatial distributions of MS WNAs and provides quantitative dependence of the wave normal angle distribution on the magnetic latitude in the inner magnetosphere, which can be readily useful in future global simulations of radiation belt particle dynamics.

Description: Abstract Significant steady but slow variability of radiation belt proton intensity, in the energy range ∼19–200 MeV and for L〈2.4, has been observed in an empirical model derived from data taken by Van Allen Probes during 2013–2019. It is compared to predictions of a theoretical model based on measured initial and boundary conditions. Two aspects of the variability are considered in detail and require adjustments to model parameters. Observed inward transport of proton intensity maxima near L=1.9 and associated increasing intensity are caused in the model by inward radial diffusion from an external source while conserving the first two adiabatic invariants. The diffusion coefficient is constrained by these observations and is required to have increased near the start of 2015 by a factor ∼2. Observed decay of proton intensity at L〈1.6 can be caused only in part by energy loss to free and bound electrons in the local plasma and neutral atmosphere. Another, unknown loss mechanism is required to match observed proton decay rates as a function of energy. Accounting for the expected influence of slow radial diffusion at low L, the additional loss should have a mean lifetime near 22 years, independent of L and energy in the range ∼19–70 MeV. Several candidate loss mechanisms are considered—added plasma or neutral density, elastic Coulomb scattering, plasma wave scattering, field‐line curvature scattering, and collision with orbital debris—but none are found viable.

Description: Abstract During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn's D ring and the denser layers of the planet's atmosphere, revealing the presence of an unanticipated low‐latitude current system via the associated azimuthal perturbation field peaking typically at ~10‐30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field‐aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field‐aligned current densities above the ionosphere are typically ~5‐10 nA m‐2, more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field‐aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north‐south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller‐scale structures, possibly due to rapid temporal or local time variations.

Description: Abstract During geomagnetic disturbances the solar wind arrives in the form of characteristic sequences lasting from tens of hours to days. The most important magnetic storm drivers are the coronal mass ejections (CMEs) and the slow‐fast stream interaction regions (SIRs). Previous data‐based magnetic field models did not distinguish between these types of the solar wind driving. In the present work we retained the basic structure of the [Tsyganenko and Andreeva, 2015] model, but fitted it to data samples corresponding to (1) SIR‐driven storms, (2) CME‐driven storms preceded with a~shock ahead of the CME, and (3) CME‐driven storms without such shocks. The storm‐time dynamics of the model current systems has been represented using the parametrization method developed by Tsyganenko and Sitnov [2005], based on dynamical variables Wi, calculated from concurrent solar wind characteristics and their previous history. The database included observations of THEMIS, Polar, Cluster, Geotail, and Van Allen Probes missions during 155 storms in 1997‐‐2016. The model current systems drastically differ from each other with respect to decay rate and total current magnitudes. During SIR‐induced storms, all current systems saturate, while during CME‐induced disturbances the saturation occurs only for the symmetric ring current (SRC) and the tail current. The partial ring current (PRC) parameters are drastically different between SIR‐ and CME‐induced storm sets. In the case of SIR‐driven storms, the total PRC is comparable with SRC, whereas for all CME‐induced events it is nearly twice higher. The results are compared with GOES 15 magnetometer observations.

Description: Abstract Whistler mode chorus waves influence the dynamics of the Earth's radiation belts and the inner magnetosphere through gyroresonant wave particle interactions. Chorus waves are generated by anisotropic hot electrons from a few to tens of keV, called source electrons, which have increased access from the nightside plasma sheet to the inner magnetosphere during geomagnetic storms. The primary drivers of geomagnetic storms are coronal mass ejections (CMEs) and co‐rotating interaction regions (CIRs). Through differences in their characteristic physical parameters they can each impact the nightside plasma sheet differently. Using Van Allen Probes observations, we have conducted a superposed epoch analysis of chorus wave activity and source electron development across all local times between L=2–6 during 25 CME‐ and 35 CIR‐driven storms. The superposed epoch analysis shows that chorus wave power follows the storm‐phase dependent access of the source electron population. Chorus waves and source electrons are observed on the dawnside during the main phase, when open drift path access via eastward convective drift from the plasma sheet is enhanced. During the recovery phase, chorus waves and source electrons are observed at all magnetic local times (MLTs) with low intensities, exemplifying the formation of a weak symmetric, trapped electron population. A linear theory approximation for wave growth from source electron observations shows that increased wave growth follows the enhanced source electrons during each storm phase. CME and CIR storms display similar behavior and levels of average wave power, however chorus wave activity reaches lower L‐shells during CME storms on average.

Description: Abstract Stable auroral red (SAR) arcs provide opportunities to study inner magnetosphere‐ionosphere coupling at mid‐latitudes. An imaging system at a single‐site obtains evidence of seasonal variations in SAR arc brightness and occurrence rates using events widely separated in time, as observed during different geomagnetic storms. The first SAR arc observed using two all‐sky imagers (ASI) at geomagnetic conjugate points described seasonal effects at the same time for the same storm (Martinis et al., 2019a). Here we report on modeling studies that enable specification of the roles of local “receptor conditions” in each hemisphere, plus the division of driving energy from a single source region into conjugate ionospheres. The geomagnetic storm of 1 June 2013 produced SAR arcs observed by conjugate ASIs yielding 73 Rayleighs (R) at Millstone Hill (L = 2.64) in the summer hemisphere and 300 R during local winter at Rothera (L = 2.92). With incoherent scatter radar (ISR) data not available to specify input conditions, we offer a new simulation approach using non‐ISR observations to specify local receptor conditions. These include a combination of semi‐empirical models (IRI and MSIS) calibrated by local ionosonde and DMSP satellite data. We find that the driving mechanism (heat conduction entering the ionosphere) is not an equal partition of energy from the ring current source region, but one that is weaker in the summer hemisphere where the local receptor conditions are poised to produce fainter SAR arcs. The relationship between SAR arcs and recently discovered STEVE events are discussed and require further study.

Description: Abstract We present midlatitude solar response and linear trend from CSU/USU Na lidar nocturnal temperature observations between 1990 and 2017. Along with the nightly‐mean temperatures (_Ngt), we also use the corresponding 2‐hour‐means centered at midnight (_2MN), resulting in vertical trend profiles similar in shapes as those previously published. The 28‐year trend from _Ngt (_2MN) dataset starts from a small warming at 85 km, to cooling at 87 (88) km, reaching a maximum of 1.85±0.53(1.09±0.74 ) at 92 (93) km, and turns positive again at 102 (100) km. The 6‐mo winter trend is much cooler than the 4‐mo summer trend with comparable solar response varying around 5±1K/100SFU throughout the profile (85‐105 km) with higher summer values. We explore the observed summer/winter trend difference in terms of observed gravity wave heat flux heating rate at a nearby station and the long‐term trend of gravity wave variance at a midlatitude. Between 89 and 100 km, the lidar trends are within the error‐bars of the Leibniz Middle Atmosphere (LIMA) summer trends (1979‐2013), which are nearly identical to the lidar‐Ngt trend. We address the need of long dataset for reliable analysis on trend, the extent of trend uncertainty due to possible tidal bias, the effect of a Pinatubo/episodic function, and the impact of stratospheric ozone recovery.

Description: Abstract In this study, we focus on the recovery phase of a geomagnetic storm that happened on 6–11 September 2017. The ground‐based total electron content data, as well as the F region in situ electron density, measured by the Swarm satellites show an interesting feature, revealing at low and equatorial latitudes on the dayside ionosphere prominent positive and negative responses at the Asian and American longitudinal sectors, respectively. The global distribution of thermospheric O/N2 ratio measured by global ultraviolet imager on board the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics satellite cannot well explain such longitudinally opposite response of the ionosphere. Comparison between the equatorial electrojet variations from stations at Huancayo in Peru and Davao in the Philippines suggests that the longitudinally opposite ionospheric response should be closely associated with the interplay of E region electrodynamics. By further applying nonmigrating tidal analysis to the ground‐based total electron content data, we find that the diurnal tidal components, D0 and DW2, as well as the semidiurnal component SW1, are clearly enhanced over prestorm days and persist into the early recovery phase, indicating the possibility of lower atmospheric forcing contributing to the longitudinally opposite response of the ionosphere on 9–11 September 2017.

Description: Abstract Incoherent scatter radars (ISR) estimate the electron and ion temperatures in the ionosphere by fitting measured spectra of ion‐acoustic waves to forward models. For radars looking at aspect angles within 5° of perpendicular to the Earth's magnetic field, the magnetic field constrains electron movement and Coulomb collisions add an additional source of damping that narrows the spectra. Fitting the collisionally narrowed spectra to collisionless forward models leads to errors or underestimates of the plasma temperatures. This paper presents the first fully kinetic particle‐in‐cell (PIC) simulations of ISR spectra with collisional damping by velocity dependent electron‐electron and electron‐ion collisions. For aspect angles between 0.5° and 2° off perpendicular, the damping effects of electron‐ion and electron‐electron collisions in the PIC simulations are the same and the resulting spectra are narrower than what current theories and models predict. For aspect angles larger than 3° away from perpendicular, the simulations with electron‐ion collisions match collisionless ISR theory well, but spectra with electron‐electron collisions are narrower than theory predicts at aspect angles as large as 5° away from perpendicular. At aspect angles less than 5° the particle‐in‐cell simulations produce narrower spectra than previous simulations using single particle displacement statistics that include both electron‐ion and electron‐electron collisions. The narrowing of spectra by electron‐electron collisions in the PIC code between 3° and 5° away from perpendicular is currently neglected when fitting measured spectra from the Jicamarca and Millstone Hill radars, leading to underestimates of electron temperatures by as much as 25% at small aspect angles.

Description: Abstract Relativistic electron flux responses in the inner magnetosphere are investigated for 28 magnetic storms driven by Corotating Interaction Region (CIR) and 27 magnetic storms driven by Coronal Mass Ejection (CME), using data from the Relativistic Electron‐Proton Telescope (REPT) instrument on board Van‐Allen Probes from Oct‐2012 to May‐2017. In this present study we analyze the role of CIRs and CMEs in electron dynamics by sorting the electron fluxes in terms of averaged solar wind parameters, L‐values, and energies. The major outcomes from our study are: (i) At L = 3 and E = 3.4 MeV, for 〉70% cases the electron flux remains stable, while at L = 5, for ~82% cases it changes with the geomagnetic conditions. (ii) At L = 5, ~53% of the CIR storms and 30% of the CME storms show electron flux increase. (iii) At a given L‐value, the tendency for the electron flux variation diminishes with the increasing energies for both categories of storms. (iv) In case of CIR driven storms, the electron flux changes are associated with changes in Vsw and Sym‐H. (v) At L ~ 3, CME storms show increased electron flux, while at L ~ 5, CIR storms are responsible for the electron flux enhancements. (vi) During CME and CIR driven storms, distinct electron flux variations are observed at L = 3 and L = 5.

Description: Abstract Magnetometers deployed on the largest satellite constellation to date are leveraged as a space‐based sensor network to study space‐time variability in auroral field‐aligned currents (FACs). The cubesat constellation of Planet Labs Inc. consists of nearly 200 satellites in two polar Sun‐synchronous orbits, with median spacecraft separations on the order of 375 km, and some occasions of opportunity providing much closer spacing. Each spacecraft contains a magnetoinductive magnetometer, able to sample the ambient magnetic field at 0.1 to 10 Hz with 〈200‐nT sensitivity. In this study, seven satellites from the Planet constellation were used to investigate space‐time variations in FACs over an active auroral display during a 10‐min interval. The aurora occurred during the early recovery phase of a geomagnetic storm and was characterized by large‐scale vortical motions and embedded rayed structure. Clear signatures of the large‐scale auroral current system were detected by the orbital magnetometers. Estimation of FAC patterns was carried out using three different methods. The results suggest a high degree of spatial and temporal variability during the 10‐min interval. The location of upward and downward current channels relative to the aurora was consistent with theoretical expectations, but current densities were not well correlated with visible features in the available imagery, suggesting unresolved small‐scale structure not captured by the collaborative observations. Advantages, limitations, and caveats in using opportunistic networks of low‐quality space‐based magnetometers to study dynamic auroral phenomena are discussed.

Description: Abstract A preliminary study on the first‐time observation of the submeter‐scale disturbances in the ionospheric E layer above the near‐equatorial site of Cochin (10.04°N, 76.33°E; Declination: −1°37′, Inclination: 7°2′), India, using an atmospheric radar in the high very high frequency range at 205 MHz has been carried out. Selected cases of E layer disturbances observed in the 90‐ to 105‐km altitude region, during summer, equinox, and winter periods are reported. It is noted that the submeter‐scale disturbances exist in the near‐equatorial E region with a life span of few minutes to several hours. Both continuous and intermittent echoes were seen in the 90‐ to 105‐km region. The continuous irregularities remain at constant altitude initially, descend and vanish in the later stage. The E layer irregularities observed in the morning and noon were continuous, whereas the echoes after the late afternoon were quasiperiodic in nature with a lifetime of 10–15 min. However, no apparent mean Doppler is observed during this period and is expected to be associated with a turbulent cascade from long wavelengths to shorter ones.

Description: Abstract Spacecraft observations within recent decades have demonstrated that ultralow frequency (ULF) waves play an important role in the dynamics of Earth's magnetosphere through drift and/or drift‐bounce resonance with different particle species. The phase relationship between resonant particles and waves can help us understand the wave‐particle interacting processes. It has been revealed that the phase relationship between the drift resonant particles and the fundamental poloidal‐mode electric field is a signature to judge the energy transfer direction between particles and waves, and the local phase space density gradient. Here we explore whether there are other physical meanings of the phase relationship for drift‐bounce resonant particles in addition to the similar ones as drift resonant particles. In this study, we find that (1) under different field‐aligned structures, the poloidal‐mode electric fields will show different phase relationship with drift‐bounce resonant particles at the magnetic equator. It indicates that the phase relationship can be used to diagnose the parallel morphologies of ULF wave electric field. (2) If particle's phase space density variations caused by resonance effect become more dominant while compared with the nonresonant effect, the phase difference becomes much closer to inphase or antiphase. (3) When wave's parallel morphology is given, the phase relationship can be used to judge the energy transfer direction between waves and particles. These findings can provide new sights into exploring ULF waves' propagation and distribution along magnetic field lines and studying their interaction with charged particles in the magnetosphere.

Description: Abstract The structure of the upper atmosphere of Mars provides insights into the physical mechanisms that drive escape of species into outer space. Deviations in plasma density profiles with altitude from the theoretical exponentially decaying formulation have been routinely observed for decades yet remain largely unexplained. Proposed mechanisms driving this variability have focused primarily on plasma‐specific processes, as limited by past plasma‐only observations. The Mars Atmosphere and Volatile Evolution mission's Neutral Gas and Ion Mass Spectrometer data set has recently provided unprecedented planetographic coverage for both ions and neutrals in the Martian upper atmosphere. Ion, electron, and neutral density profiles with altitude, collected on the sun‐lit inbound portion of the spacecraft orbit have been analyzed. It was found that neutral species, measured between ~160 and 200 km, behave consistently with the bulk atmosphere and that variations in ion density profiles follow neutral profile variations at the same altitudes in 70% of the observations. In the remaining 30%, additional structure was apparent in the ionized species' profiles that were found to preferentially lie in regions of strong crustal field or to be measured near dawn. A 1‐D ionospheric model was used to show that many observed features in plasma profiles are directly driven by neutral atmospheric features, providing strong evidence for ion‐neutral coupling in the atmosphere of Mars.

Description: Abstract Predictive models for the Earth's space environment routinely use parameters from the solar wind as inputs. Measurements from spacecraft orbiting the first Lagrange point serve as convenient values for these inputs. The mass, momentum, and energy input into the Earth's space environment, however, are a function of the shocked and processed plasma within the magnetosheath, which can vary significantly from the pristine solar wind at the first Lagrange point. Here statistical measurements from the OMNI data set are combined with measurements by the THEMIS mission within the magnetosheath to generate uncertainty values for pressure and magnetic clock angle. These uncertainties are generated to account for known physical processes in the foreshock and magnetosheath as well as the position of the spacecraft being used to generate the OMNI data set.

Description: Abstract Polar cap patches are large sporadic enhancements of plasma density on the scale of hundreds of kilometers, which can impact the performance of Global Navigation Satellite Systems. Lagrangian Coherent Structures (LCSs) are ridges that show areas of maximal separation in a time‐evolving flow. Previous work based on modeled ionospheric flow showed that LCSs exist in the ionosphere and are barriers governing patch formation. In this work, we identify the first data‐driven LCSs in the high‐latitude ionosphere using Super Dual Auroral Radar Network (SuperDARN) ion convection fields. The LCSs found using the Ionosphere‐Thermosphere Algorithm for LCSs are compared during geomagnetically quiet and active periods. The shape of the LCS is found to be dependent on the electric potential pattern. A consistent two‐cell pattern results in a W‐shaped LCS, but when the two‐cell pattern breaks down, the LCS loses this characteristic shape. The changes in the electric potential, and thus the LCS, are likely due to changes in the interplanetary magnetic field. A comparison between LCSs obtained from empirical models and data reveal that the data‐driven LCSs are poleward of and have a shorter longitudinal span than the model‐based LCSs. A comparison of the LCS location and the formation of a polar cap patch on 17 March 2015 showed that the center of the patch developed from plasma on the main LCS ridge, and this is confirmed with a separate polar cap patch event from 26 September 2011.

Description: Abstract A Pc5 wave was simultaneously observed in the ionosphere by EKB radar and in the magnetosphere by both Van Allen Probe spacecraft within a substorm activity. The wave was located in the nightside, in 1.5‐ to 3‐hr magnetic local time sector, and in the region corresponding to the magnetic shells with maximal distances 4.6–7.8 Earth's radii. As it was found using both the radar and spacecraft data, the wave had frequency of about 1.8 mHz and azimuthal wave number m≈−10; that is, the wave was westward propagating. The EKB radar data revealed the equatorward wave propagating in the ionosphere, which corresponded to the earthward propagation in the magnetosphere. Furthermore, the field‐aligned magnetic component was approximately 2 times larger than both transverse components and accompanied by antiphase pressure oscillations; that is, the wave is compressional and diamagnetic. According to both radar and spacecraft measurements, among two transverse magnetic components, the dominant one was the poloidal. The wave was possibly driven by substorm‐injected energetic protons registered by the spacecraft: the proton fluxes were modulated with the wave frequency at energies of about 90 keV, which corresponded to the energy of the drift wave‐particle resonance. The wave frequency was much lower than the minimal frequency of the field line resonance calculated using the spacecraft data. We conclude that the wave is not the Alfvén mode, but some kind of compressional wave, for example, the drift‐compressional mode.

Description: Abstract The primary focus of this study was the motion of auroral patches and the polarization electric field generated therein observed on 9 November 2015 in an experiment using the European incoherent scatter (EISCAT) radars, Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA), and an all‐sky imager simultaneously. Based on the all‐sky imager data, the drift speed of the auroral patches corresponded to a southward electric field of 14.1( ± 3.7)–17.2( ± 4.5) mV/m. The convective electric field derived from the EISCAT radars and KAIRA observation was approximately 14.6 mV/m in the southward direction. This suggest that the spatial distribution of the auroral patches reflects the distribution of the cold plasma in the magnetosphere. The electron density and the height‐integrated Hall conductance between 80 and 120 km were enhanced by a factor of 2–4 inside the auroral patches. In this situation, a polarization electric field was generated therein. Enhanced ion velocities due to the polarization electric field was observed at up to 200‐km altitude; however, the absolute values of the ion velocities were approximately 40% of what was expected from the polarization electric field. A field‐aligned current (FAC) from 5 to 10 μA/m−2 in the edges of the auroral patches could explain the weakening of the polarization electric field. Since a FAC of that order of magnitude corresponded with that observed by the Swarm satellite, it was suggested that the polarization electric field was weakened by the FAC. Furthermore, the polarization electric field propagated upward from the dynamo region to at least 200 km.

Description: Abstract Velocity distribution functions (VDFs) are a key to understanding the interplay between particles and waves in a plasma. Any deviation from an isotropic Maxwellian distribution may be unstable and result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer (MAG) onboard Venus Express, we study proton distributions in the plasma environment of Venus. We focus on the temperature anisotropy, that is, the ratio between the proton temperature perpendicular (T⊥) and parallel (T‖) to the background magnetic field. We calculate average values of T⊥ and T‖ for different spatial areas around Venus. In addition we present spatial maps of the average of the two temperatures and of their average ratio. Our results show that the proton distributions in the solar wind are quite isotropic, while at the bow shock stronger perpendicular than parallel heating makes the downstream VDFs slightly anisotropic (T⊥/T‖ 〉 1) and possibly unstable to generation of proton cyclotron waves or mirror mode waves. Both wave modes have previously been observed in Venus's magnetosheath. The perpendicular heating is strongest in the near‐subsolar magnetosheath (T⊥/T‖≈3/2), which is also where mirror mode waves are most frequently observed. We believe that the mirror mode waves observed here are indeed generated by the anisotropy. In the magnetotail we observe planetary protons with largely isotropic VDFs, originating from Venus's ionosphere.

Description: Abstract Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) simulations are diagnostically analyzed to investigate the causes of mesosphere and lower thermosphere (MLT) wind changes at middle latitudes during the 17 April 2002 storm. In the early phase of the storm, middle‐latitude upper thermospheric wind changes are greater and occur earlier than MLT wind changes. The horizontal wind changes cause downward vertical wind changes, which are transmitted to the MLT region. Adiabatic heating and heat advection associated with downward vertical winds cause MLT temperature increases. The pressure gradient produced by these temperature changes and the Coriolis force then drive strong equatorward meridional wind changes at night, which expand toward lower latitudes. Momentum advection is minor. As the storm evolves, the enhanced MLT temperatures produce upward vertical winds. These upward winds then lead to a decreased temperature, which alters the MLT horizontal wind pattern and causes poleward wind disturbances at higher latitudes.

Description: Abstract We present results from the ionospheric heating experiment conducted at the High Frequency Active Auroral Research Program (HAARP) facility, Alaska, on 12 March 2013. During the experiment, HAARP transmitted in the direction of the magnetic zenith X‐mode 4.57‐MHz wave. The transmitted power was modulated with the frequency of 0.9 mHz, and it was pointed on a 20‐km spot at the altitude of 120 km. The heating (1) generates disturbances in the magnetic field detected with the fluxgate magnetometer on the ground and (2) produces bright luminous spots in the ionosphere, observed with the HAARP telescope. Numerical simulations of the 3‐D reduced magnetohydrodynamic (MHD) model reveal that these effects can be related to the magnetic field‐aligned currents, excited in the ionosphere by changing the conductivity in the E region when the large‐scale electric field exists in the heating region.

Description: Abstract Seasonal and source variations of migrating and nonmigrating tides are studied using Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics‐Sounding of the Atmosphere using Broadband Emission Radiometry temperature data at 10°N (5–15°N) for the year 2009. The migrating DW1 shows equinoctial maximum and summer minimum at low latitudes. It shows equinoctial asymmetry with larger amplitudes during spring equinox than fall equinox. The migrating semidiurnal tidal amplitude (SW2) shows larger amplitudes (~20 K) during March–October at 30–60°S. Its seasonal variation resembles stratospheric (10 hPa) ozone variations at southern midlatitudes. During the sudden stratospheric warming of 2009, the SW1 shows larger amplitudes over the equator and it is generated due to nonlinear interaction between SW2 and planetary wave of zonal wave number 1. The eastward nonmigrating DE4 and DE3 tides enhance in summer. The DE3 and DE4 appear to be generated due to latent heat release in the troposphere, as their amplitudes in the National Center for Environmental Prediction (NCEP)'s Precipitable water vapor (proxy for latent heat release) enhance at similar times as in mesosphere. The DW2 and DW0 tides are likely to be generated due to nonlinear interaction between DW1 and planetary wave of zonal wave number 1. The SW3 enhancement during the early winter (November‐December) may be due to nonlinear interaction between DW1 and the large‐amplitude DW2. The nonlinear interactions of DW1 with planetary wave and nonmigrating tides explain the summer minimum and equinoctial asymmetry of DW1.

Description: Abstract No abstract is available for this article.

Description: Abstract Fast magnetosonic (MS) waves can play an important role in the evolution of the inner magnetosphere. However, there is still not an effective method to quantitatively identify such waves for observations of the Van Allen Probes reasonably. In this paper, we used Van Allen Probes data from 18 September 2012 to 30 September 2014 to find a more comprehensive automatic detection algorithm for fast MS waves through statistical analysis of the major properties, including the planarity, ellipticity, and wave normal angle of whole fluctuations using the singular value decomposition method. According to a control variate method, we find an obvious difference between fast MS waves and other waves in the statistical distribution of their major properties. After eliminating the influence of background noises, by excluding fluctuations at L 〈 1.8, we set up an automatic detection algorithm applied to fast MS waves, that is, smaller than 0.2 for the absolute value of wave ellipticity, larger than 70° for the wave normal angle, with frequency range of 2 Hz to 1.5 fLHR (fLHR is the local lower hybrid resonance frequency). Finally, we have checked the planarity to verify availability of this method and tested this completely automatic method on the Van Allen Probes data and found some results consistent with previous studies. Inside the plasmapause, we found that there is a more obviously favorable occurrence of MS waves at dusk sector with increasing magnetic latitudes.

Description: Abstract Wave‐like structures in the upper atmospheric nightglow brightness were observed on the night of 22 August 2017, approximately 8 hr following a total solar eclipse. These wave‐like perturbations are signatures of atmospheric gravity waves and associated traveling ionospheric disturbances (TIDs). Observations were made in the red line (OI 630.0 nm) and the green line (OI 557.7 nm) from Carbondale, IL, at 2–10 UTC on 22 August 2017. Based on wavelet analyses, the dominant time period in both the red and green lines was around 1.5 hr. Differential total electron content data obtained from Global Positioning System total electron content measurements at Carbondale, IL, and ionospheric parameters from digisonde measurements at Idaho National Laboratory and Millstone Hill showed a similar dominant time period. Based on these observations and their correlation with geomagnetic indices, the TIDs appear to be associated with geomagnetic disturbances. In addition, by modeling the ionosphere‐thermosphere system's response to the eclipse, it was seen that while the eclipse enhanced the O/N2 ratio and electron density (Ne) at 250 km during our observation period, it did not affect the TIDs. Vertical (7 m/s) and meridional (616 m/s) phase velocities of the TIDs were estimated using cross‐correlation analysis between red and green line brightness profiles and spectral analysis of the differential total electron content keogram, respectively. This provides a method to characterize the three‐dimensional wave properties of TIDs.

Description: Abstract During 30 September to 9 October 2016, Hurricane Matthew traversed the Caribbean Sea to the east coast of the United States. During its period of greatest intensity, in the central Caribbean, Matthew excited a large number of concentric gravity waves (GWs or CGWs). In this paper, we report on hurricane‐generated CGWs observed in both the stratosphere and mesosphere from spaceborne satellites and in the ionosphere by ground Global Positioning System receivers. We found CGWs with horizontal wavelengths of ~200–300 km in the stratosphere (height of ~30–40 km) and in the airglow layer of the mesopause (height of ~85–90 km), and we found concentric traveling ionospheric disturbances (TIDs or CTIDs) with horizontal wavelengths of ~250–350 km in the ionosphere (height of ~100–400 km). The observed TIDs lasted for more than several hours on 1, 2, and 7 October 2016. We also briefly discuss the vertical and horizontal propagation of the Hurricane Matthew‐induced GWs and TIDs. This study shows that Hurricane Matthew induced significant dynamical coupling between the troposphere and the entire middle and upper atmosphere via GWs. It is the first comprehensive satellite analysis of gravity wave propagation generated by hurricane event from the troposphere through the stratosphere and mesosphere into the ionosphere.

Description: Abstract For the first time an evaluation of the whistler rate around the Earth is performed using results from the neural network aboard the microsatellite DEMETER. It is shown that the rate of whistlers with low dispersion calculated all around the Earth as a function of longitude vary between 1 and 6 s−1 during nighttime (22.30 LT) and between 0.5 and 0.7 s−1 during daytime (10.30 LT). The whistler rate is anticorrelated with the F10.7‐cm solar flux. A decrease by 25% of the solar flux corresponds to an increase of 62% (26%) of the averaged whistler rate calculated for the entire Earth during nighttime (daytime). Using this averaged whistler rate, the global lightning rate is estimated to be of the order of 123 s−1 (27 s−1) during nighttime (daytime). The main conclusion concerns the precipitation of the electrons in the radiation belt by interaction with the whistlers. It is shown that the decrease of the lightning activity at solar minimum (shown with the help of the Schumann resonances) is largely counterbalanced by the increase of the whistler rates in the upper part of the ionosphere due to the decrease of the ionospheric absorption.

Description: Abstract Geographical and seasonal variations of mesospheric bores were derived from mesospheric airglow observations by the Visible and near Infrared Spectral Imager (VISI) of Ionosphere, Mesosphere, upper Atmosphere and Plasmasphere (IMAP) mission onboard the International Space Station. In the three‐year data set spanning September 2012 to August 2015, 306 mesospheric bore events were found between 55ºS and 55ºN in the O2(0‐0) airglow whose peak height is around 95 km. The distribution of the bore events showed a high occurrence at equatorial latitudes especially during the equinox seasons and at winter midlatitudes. These latitudes and seasons are also known for being the place and time where the migrating diurnal and semidiurnal tides have a large temperature amplitude at the upper mesosphere altitude. This coincidence suggests that the majority of mesospheric bores occurred in a temperature inversion layer, which is related to the tides. The local time variation of the bore occurrences at midlatitudes showed a minimum around midnight. The local time variation at equatorial latitudes is more widely distributed compared to those at midlatitudes. The dominant propagation direction of mesospheric bores is from the winter hemisphere to the summer hemisphere.

Description: Abstract In this paper, we presented two observational cases and simulations to indicate the relationship between the formation of butterfly‐like electron pitch angle distributions and the emission of low‐harmonic (LH) fast magnetosonic (MS) waves inside the high‐density plasmasphere. In the wave emission region, the pitch angle of relativistic (〉1 MeV) electrons becomes obvious butterfly‐like distributions for both events (near‐equatorially mirroring electrons are transported to lower pitch angles). Unlike relativistic (〉1 MeV) electrons, energetic electrons (〈1 MeV) change slightly, except that relatively low‐energy electrons (〈~150 keV) show butterfly‐like distributions in the 21 August 2013 event. In theory, the LH MS waves can affect different‐energy electrons through the bounce resonance, Landau resonance, and transit time scattering. By performing the Fokker‐Planck diffusion simulations, we demonstrate that the bounce resonance with the LH MS waves mainly leads to the butterfly pitch angle distribution of MeV electrons, whereas the Landau resonance and transit time scattering mainly affect energetic electrons in the high‐density region.

Description: Abstract In the Jovian magnetosphere, sulfur and oxygen ions supplied by the satellite Io are distributed in the so‐called Io plasma torus. The plasma torus is located in the inner area of the magnetosphere and the plasma in the torus corotates with the planet. The density and the temperature of the plasma in the torus have significant azimuthal variations. In this study, data from three‐year observations obtained by the Hisaki satellite, from December 2013 to August 2016, were used to investigate statistically the azimuthal variations and to find out whether the variations were influenced by the increase in neutral particles from Io. The azimuthal variation was obtained from a time series of sulfur ion line ratios, which were sensitive to the electron temperature and the sulfur ion mixing ratio S3+/S+. The major characteristics of the azimuthal variation in the plasma parameters were consistent with the dual hot electron model, proposed to explain previous observations. On the other hand, the Hisaki data showed that the peak System III longitude in the S3+/S+ ratio was located not only around 0°–90°, as in previous observations, but also around 180°–270°. The rotation period, the System IV periodicity, was sometimes close to the Jovian rotation period. Persistent input of energy to electrons in a limited longitude range of the torus is associated with the shortening of the System IV period.