ALBERT

Description: Magnetospheric ions with energies less than tens of eV originate from the ionosphere. Positive low-energy ions are complicated to detect onboard sunlit spacecraft at higher altitudes, which often become positively charged to several tens of volts. We use two Cluster spacecraft and study low-energy ions with a technique based on the detection of the wake behind a charged spacecraft in a supersonic ion flow. We find that low-energy ions usually dominate the density and the outward flux in the geomagnetic tail lobes during all parts of the solar cycle. The global outflow is of the order of 10 26 ions/s and often dominates over the outflow at higher energies. The outflow increases by a factor of 2 with increasing solar EUV flux during a solar cycle. This increase is mainly due to the increased density of the outflowing population, while the outflow velocity does not vary much. Thus, the outflow is limited by the available density in the ionospheric source, rather than by the energy available in the magnetosphere to increase the velocity.

Description: We analyze data acquired by the Van Allen Probes on 8 November 2012, during a period of extended low geomagnetic activity, to gain new insight into plasmaspheric ultra-low-frequency (ULF) waves. The waves exhibited strong spectral power in the 5–40 mHzband and included multiharmonic toroidal waves visible up to the 11th harmonic, unprecedented in the plasmasphere. During this wave activity, the interplanetary magnetic field cone angle was small, suggesting that the waves were driven by broadband compressional ULF waves originating in the foreshock region. This source mechanism is supported by the tailward propagation of the compressional magnetic field perturbations at a phase velocity of a few hundred kilometers per second that is determined bythe cross phase analysis of data from the two spacecraft. We also find that the coherence and phase delay of the azimuthal components of the magnetic field from the two spacecraft strongly depend on the radial separation of the spacecraft and attribute this feature to field line resonance effects. Finally, using the observed toroidal wave frequencies, we estimate the plasma mass density for L  = 2.6–5.8. By comparing the mass density with the electron number density that is estimated from the spectrum of plasma waves, we infer that the plasma was dominated by H + ions and was distributed uniformly along the magnetic field lines. The electron density is higher than the prediction of saturated plasmasphere models, and this “super saturated” plasmasphere and the uniform ion distribution are consistent with the low geomagnetic activity that prevailed.

Description: To accurately quantify the effect of Solar Proton Events (SPEs) on the atmosphere requires a good estimate of the particle energy deposition in the middle atmosphere (60–100 km) and how the energy is distributed globally. Protons in the energy range 1–20 MeV, depositing their energy in the middle atmosphere, are subject to more complex dynamics with strong day-night asymmetries compared to higher energy particles. Our study targets 6 SPEs from 2003 to 2012. By using measurements from the Medium Energy Proton and Electron Detector (MEPED) on all available Polar Orbit Environment Satellites (POES), we show that in the main phase of geomagnetic storms the dayside cutoff latitudes are pushed poleward, while the nightside cutoff latitudes have the opposite response, resulting in strong day-night asymmetries in the energy deposition. These features cannot be measured by the frequently used Geostationary Operational Environmental Satellites (GOES). Assuming that the protons impact the polar atmosphere homogeneously above a fixed nominal latitude boundary will therefore give a significant overestimate of the energy deposited in the middle atmosphere during SPEs. We discuss the magnetospheric mechanisms responsible for the local time response in the cutoff latitudes and provide a simple applicable parameterization which includes both day- and nightside cutoff latitude variability using only the Dst, the northward component of the interplanetary magnetic field, and solar wind pressure. The parameterization is utilized on the GOES particle fluxes and the resulting energy deposition successfully captures the day-night asymmetry in good agreement with the energy deposition predicted from the POES measurement.

Description: We investigate local magnetospheric processes governing the morphology and variability of Ganymede's aurora depending on its position with respect to the center of the Jovian plasma sheet. We couple an existing three-dimensional multifluid simulation to a new aurora brightness model developed for this study. With this, we are able to qualitatively and quantitatively show that the short- and long-period variabilities observed in Ganymede's auroral footprint at Jupiter are also predicted to be present in the brightness and morphology of the aurora at Ganymede. We also examine the relationship between acceleration structures and precipitation of electrons in Ganymede's neutral atmosphere by looking at the component of the electric field parallel to Ganymede's magnetic field. Our results confirm that regions of electron accelerations coincide with regions of brightest auroral emissions, as expected. Finally, we identify the likely source regions of electrons generating the aurora at Ganymede and discuss the plasma dynamic mechanisms likely responsible for these accelerations.

Description: The spatially inhomogeneous, small-scale crustal magnetic fields of Mars influence the escape of planetary atmospheric species and the interaction of the solar wind with the ionosphere. Understanding the plasma response to crustal magnetic field regions can therefore provide insight to ionospheric structure and dynamics. To date, several localized spatial structures in ionospheric properties that have been observed over regions of varying magnetic field at Mars have yet to be explained. In this study, a two-dimensional ionospheric model is used to simulate the effects of field-aligned plasma transport in regions of strong crustal magnetic fields. Resulting spatial and diurnal plasma distributions are analyzed and found to agree with observations from several spacecraft, and offer compelling interpretations for many of the anomalous ionospheric behaviors observed at or near regions of strong crustal magnetic fields on Mars.

Description: It is well-established that large Solar Energetic Particle (SEP) events affect ozone in the middle atmosphere through chemical reactions involving odd hydrogen (HOx) species. We analyze global middle atmospheric effects at local nighttime for two large SEP events during the intervals of 7-17 November 2004 and 20-30 August 2005. Properties of the SEP events and concomitant geomagnetic storms are discussed using in situ measurements. Temporal dynamics and latitudinal distribution of HOx and ozone densities inferred from measurements by the AURA/MLS instrument are analyzed. We show statistically significant increases of nightime hydroxyl (OH) density in the middle atmosphere up to 5°10 6  cm -3 in the latitude range from 70° down to 50° latitude in northern and to -40° latitude in southern hemispheres in connection with peaks in proton fluxes of 〉10 MeV energy range measured by GOES spacecraft. During the storm main phases the nighttime OH density increases were observed around ±50° in southern and northern hemispheres in the altitude range of 65-80 km. There is a correspondence between averaged nighttime OH partial column density (in 0.005 to 0.1 hPa pressure range) in the polar latitudes and energetic proton (〉10 MeV) fluxes. Corresponding statistically significant nighttime ozone destructions up to 45% are observed from 70° down to 60° latitude in the northern and southern hemispheres. The SEP impulsive phases correspond to onsets of ozone density depletions. Larger relative ozone destructions are observed in the northern hemisphere in November and in the southern hemisphere in August. Simultaneous measurements of ozone density by the TIMED/SABER instrument independently confirm the MLS results.

Description: We analyze localized ionospheric – thermospheric (IT) events in response to external driving by a high-speed stream (HSS) during the ascending phase of the Solar Cycle 24. The HSS event occurred from ~ 29 April to 5 May, 2011. The HSS itself (and not the associated co-rotating interaction region) caused a moderate geomagnetic storm with peak SYM-H = -55 nT and prolonged auroral activity. We analyze TIMED/SABER measurements of nitric oxide (NO) cooling emission during the interval as a measure of thermospheric response to auroral heating. We identify several local cooling emission (LCE) events in high- to sub-auroral latitudes. Individual cooling emission profiles during these LCE events are enhanced at ionospheric E layer altitudes. For the first time, we present electron density profiles in the vicinity of the LCE events using collocated COSMIC radio-occultation (RO) measurements. Measurements at local nighttime show the formation of an enhanced E-layer (about 2.5 times increase over the undisturbed value) at ≥100 km altitude. Daytime electron density profiles show relatively smaller enhancements in the E-layer. We suggest that the IT response is due to additional ionization caused by medium energy electron (〉10 keV) precipitation into the sub-auroral to high-latitude atmosphere associated with geomagnetic activity during the HSS event.

Description: Several recent studies suggest that magnetic reconnection is able to erode substantial amounts of the outer magnetic flux of interplanetary magnetic clouds (MCs) as they propagate in the heliosphere. We quantify and provide a broader context to this process, starting from 263 tabulated interplanetary coronal mass ejections (ICMEs), including MCs, observed over a time period covering 17 years and at a distance of 1 AU from the Sun with Wind (1995-2008) and the two STEREO (2009-2012) spacecraft. Based on several quality factors, including careful determination of the MC boundaries and main magnetic flux rope axes, an analysis of the azimuthal flux imbalance expected from erosion by magnetic reconnection was performed on a subset of 50 MCs. The results suggest that MCs may be eroded at the front or at rear and in similar proportions, with a significant average erosion of about 40 % of the total azimuthal magnetic flux. We also searched for in situ signatures of magnetic reconnection causing erosion at the front and rear boundaries of these MCs. Nearly ~30 % of the selected MC boundaries show reconnection signatures. Given that observations were acquired only at 1 AU and that MCs are large-scale structures, this finding is also consistent with the idea that erosion is a common process. Finally, we studied potential correlations between the amount of eroded azimuthal magnetic flux and various parameters such as local magnetic shear, Alfvén speed, and leading and trailing ambient solar wind speeds. However, no significant correlations were found, suggesting that the locally observed parameters at 1 AU are not likely to be representative of the conditions that prevailed during the erosion which occurred during propagation from the Sun to 1 AU. Future heliospheric missions, and in particular Solar Orbiter or Solar Probe Plus, will be fully geared to answer such questions.

Description: Sudden enhancement of the plasma pressure in the near-Earth plasma sheet is one of the common manifestations of the substorms, and is thought to play an important role in relevant disturbances in the magnetosphere and ionosphere. On 1 March 2008 four of the THEMIS (Time History of Events and Macroscale Interactions during Substorms) probes observed the sudden enhancement of the plasma pressure around 15:40 UT. The four probes were almost aligned along the Sun-Earth line, which was suitable for investigating spatial-temporal evolution of the near-Earth plasma sheet around the substorm onset. The four probes were located off the equatorial plane, according to a magnetic field model. The plasma pressure suddenly increased at the inner most probe first (at ~7.2 Re), followed by the outer probes (at ~7.5, ~8.3, and ~10.4 Re), that could be seen as a tailward propagation (or retreat) of high-pressure region (HPR). After comparing with results of a global magnetohydrodynamics (MHD) simulation, we found that only the tailward propagation of the HPR could be seen at off-equator. Near the equatorial plane, the HPR propagates earthward from the magnetotail region, then it retreats tailward. In the course of the tailward retreat, the HPR also propagates away from the equatorial plane. As a consequence, the inner most probe observed the pressure enhancement first, followed by the outer probes. The propagation of the HPR in the Z GSM direction is understood to be a combination of the convergence of the plasma flow (the divergence of bulk velocity along the Z GSM axis), and the pressure gradient force.

Description: In the region between L = 2 to 7 at all MLTs, plasmaspheric hiss was detected 38% of the time. In the limited region of L = 3 to 6 and 15 to 21 MLT (dusk sector), the wave percentage detection was the highest (51%). The latter plasmaspheric hiss is most likely due to energetic ~10-100 keV electrons drifting into the dusk plasmaspheric bulge region. On average plasmaspheric hiss intensities are an order of magnitude larger on the dayside than on the nightside. Plasmaspheric hiss intensities are considerably more intense and coherent during high solar wind ram pressure intervals. A hypothesis for this is generation of dayside chorus by adiabatic compression of preexisting 10–100 keV outer magnetospheric electrons in minimum B pockets plus chorus propagation into the plasmasphere. In large solar wind pressure events, plasmaspheric hiss can also be generated inside the plasmasphere as well. These new generation mechanism possibilities are in addition to the well-established mechanism of plasmaspheric hiss generation during substorms and storms. Plasmaspheric hiss under ordinary conditions is of low coherency, with small pockets of several cycles of coherent waves. During high solar wind ram pressure intervals (positive SYM-H intervals, plasmaspheric hiss and large L hiss can have higher intensities and be coherent. Plasmaspheric hiss in these cases is typically found to be propagating obliquely to the ambient magnetic field with θ kB0  ~ 30° to 40°. Hiss detected at large L has larger amplitudes and propagates obliquely to the ambient magnetic field (θ kB0  ~ 70°) with 2:1 ellipticity ratios.