ALBERT

Description: [1]  Neutron monitors have recorded the flux of high energy Galactic cosmic rays for more than half a century. During the recent prolonged, deep minimum in solar activity many sources indicate that modulated Galactic cosmic rays have attained new space-age highs. However reported neutron monitor rates are ambiguous; some record new highs while others do not. This work examines the record of 15 long-running neutron monitors to evaluate cosmic ray fluxes during the recent extraordinary solar minimum in a long-term context. We show that ground-level neutron rates did reach a historic high during the recent solar minimum, and we present a new analysis of the cosmic ray energy spectrum in the year 2009 versus year 1987. To do this we define a reference as the average of eight high-latitude neutron monitors, four in the northern hemisphere (Apatity, Inuvik, Oulu, Thule) and four in the southern hemisphere (Kerguelen, McMurdo, Sanae, Terre Adelie). Most stations display changes in sensitivity, which we characterize by a simple linear trend. After correcting for the change in sensitivity, a consistent picture emerges. With our correction all stations considered display new highs at the recent solar minimum, approximately 3% above the previous record high. These increases are shown to be consistent with spacecraft observations.

Description: [1]  We present the occurrence frequency of downgoing auroral electron beams in magnetic local time and invariant latitude, and the dependence on solar cycle, as indicated by F10.7, on whether the ionospheric footpoint of the satellite is illuminated or dark, and on the energy flux carried by the electrons. As previously reported, we find that the occurrence of electron beams peaks in the pre-midnight local time sector and that solar illumination at the footpoint (solar zenith angle) reduces both the occurrence and energy of the electron beams. The effect of solar maximum conditions (indicated by F10.7) is almost as large as the effect of the solar zenith angle. The characteristic energy of the electron beams is dependent on the energy flux carried, in addition to both solar zenith angle and F10.7. The beam energy (and therefore the parallel potential drop) is ~1.6 times higher for during solar minimum than during solar maximum for both dark and illuminated footpoints. The beam energy during dark solar minimum conditions is a factor of ~3 more than during sunlit minimum conditions. The ‘area’ covered by intense aurora is also reduced during solar maximum, for both sunlit and dark conditions. There is no evidence that the statistical results are due to the fact that acceleration via parallel electric fields moves to lower latitudes during solar maximum.

Description: [1]  In this paper we describe and quantify the energy transfer, flow and distribution. Our high-resolution data-set covers 13 years of OMNI, SuperMAG and Kyoto data. We employ what we consider to be the best estimates for energy sinks, and relate these to SuperMAG indices for better coverage and spatial resolution. For the energy input we have used the method of dimensional analysis [ Vasyliunas et al ., 1982] that is presented in unit power and makes it appropriate for energy analysis. A cross-correlation analysis parameterizes the magnetospheric response on the solar wind parameters during a wide range of conditions, ranging from substorms and storms up to a decade. The determined functional form is then evaluated and scaled using superposed epoch analysis of geomagnetic storms, revealing that the effective area of interaction can not be considered static. Instead we present a dynamic area which depends to the first order on the cube of the IMF B z component. Also, we find that for longer time periods this area must be increased compared to the area used for geomagnetic storms. We argue that some of the terms in the energy coupling function are contributory to describing magnetosheath conditions, and discuss how our coupling function can be related to Maxwell stress components. Also, we quantify the relative importance of the different energy sinks during substorms, geomagnetic storms and long time series, and present the coupling efficiency of the solar wind. Our energy coupling functions is compared with the ɛ parameter [ Akasofu and Perreault , 1978] and performs better for almost any event.

Description: [1]  Plasma sheet flow bursts have been suggested to correspond to different types of auroral activity, such as poleward boundary intensifications (PBIs), ensuing auroral streamers, and substorms. The flow-aurora association leads to the important question of identifying the magnetotail source region for the flow bursts and how this region depends on magnetic activity. The present study uses the ARTEMIS spacecraft coordinated with conjugate ground-based auroral imager observations to identify flow bursts beyond 45 R E downtail and corresponding auroral forms. We find that quiet-time flows are directed dominantly earthward with a one-to-one correspondence with PBIs. Flow bursts during the substorm recovery phase, and during steady magnetospheric convection (SMC) periods are also directed earthward, and these flows are associated with a series of PBIs/streamers lasting for tens of minutes with similar durations to that of the series of earthward flows. Pre-substorm onset flows are also earthward and associated with PBIs/streamers. The earthward flows during those magnetic conditions suggest that the flow bursts, which lead to PBIs and streamers, originate from further downtail of ARTEMIS, possibly from the distant tail neutral line (DNL) or tailward-retreated near-Earth neutral line (NENL) rather than from the nominal NENL location in the mid-tail. We find that tailward flows are limited primarily to the substorm expansion phase. They continue throughout the period of auroral poleward expansion, indicating that the expansion-phase flows originate from the NENL and that NENL activity is closely related to the auroral expansion of the substorm expansion phase.

Description: [1]  Precipitation extremes are expected to increase in a warming climate, thus it is essential to characterise their potential future changes. Here we evaluate eight high-resolution Global Climate Model simulations in the twentieth century and provide new evidence on projected global precipitation extremes for the 21 st century. A significant intensification of daily extremes for all seasons is projected for the mid and high latitudes of both hemispheres at the end of the present century. For the subtropics and tropics, the lack of reliable and consistent estimations found for both the historical and future simulations might be connected with model deficiencies in the representation of organised convective systems. Low inter-model variability and good agreement with high-resolution regional observations are found for the twentieth century winter over the Northern Hemisphere mid and high latitudes.

Description: [1]  Repeated observations at the subpolar front west of the Mid-Atlantic Ridge show a link between the position of the North Atlantic Current (NAC), and the spatial distribution of finescale variance, diapycnal diffusivity, and integrated energy dissipation. Observational data were collected during three cruises. A single branch of the NAC was found in 2008, approximately at 50 ∘ 30Õ N, and two branches were observed in 2010 and 2011, with alternating intensities. Shear variance was elevatedbelow the core of the NAC in all cases, resulting in an average diapycnal diffusivity that is higher by a factor of 3 compared to the averages north and south of the NAC. The integrated energy dissipation has maxima at or south of the fronts; background dissipation is highest during 2008, also the year with the highest surface eddy kinetic energy.

Description: [1]  The tropospheric response to sudden stratospheric warmings (SSWs) is analyzed in an idealized model setup regarding the respective roles of planetary-scale and synoptic-scale waves. The control model run includes a full interactive wave spectrum, while a second run includes interactive planetary-scale waves but only the time-mean synoptic-scale wave forcing from the control run. In both runs the tropospheric response is characterized by the negative phase of the respective tropospheric annular mode. But given their different latitudinal structure, the control run shows the expected response, i.e. an equatorward shift of the tropospheric jet, whereas the response in the absence of interactive synoptic eddies is characterized by a poleward jet shift. This opposite jet shift is associated with a different planetary wave variability that couples with the zonal flow between the stratosphere and the surface. These results indicate that the synoptic eddy feedback is necessary for the observed tropospheric response to SSWs.

Description: [1]  The solar eclipse on 15 January 2010 traversed Asia and completed its travel on the Shandong Peninsula in China at sunset. Two vertical-incidence ionosondes at Wuhan and Beijing and the oblique-incidence ionosonde network in North China were implemented to record the ionospheric response to the solar eclipse. Following the initial electron density decrease caused by the eclipse, the ionosphere was characterized by a strong pre-midnight enhancement, and a subsequent ionospheric decay, and a ~10 hour later post-midnight enhancement. Neither geomagnetic disturbance occurred during the eclipse day, nor did obvious nighttime peak appear for the ten-day mean of the F2-layer critical frequency ( fo F2). The electron density profilogram of the Beijing ionosonde indicates that the two enhancements were the result of the plasma flux downward from the top ionosphere, possibly due to the steep decrease of the ionospheric electron density and plasma temperature during the solar eclipse. The two-dimensional differential fo F2 maps present the regional variations of the nighttime electron density peaks and decay. Both the pre- and post-midnight enhancements initially appeared in a belt almost in parallel with the eclipse track and then drifted southward. The different magnitudes of greatest eclipse in the umbra and outside tend to account for the different occurrence times of the plasma flux. The ionospheric decay following the pre-midnight enhancement is also considered as a consequence of the eclipse shade.

Description: [1]  We study a statistics of ∂  B z /∂  x in a thin stretching current sheet (substorm growth phases) observed by Cluster between 8 and 18 R E downtail. After 2005 spacecraft separation allowed to measure directly this derivative of B z along the tail axis. The near-tail events (within 14 R E ) exhibited a straight decrease of an initially large positive ∂  B z /∂  x to ∼ 1–2 nT/ R E . In the more stretched middle tail, usually the small | ∂  B z /∂  x | 〈 0.5 − 1 nT/ R E had no clear trend and fluctuated around zero with time scales 5–15 min. In general, negative ∂  B z /∂  x were ubiquitous. At some onsets larger negative ∂  B z /∂  x  〈 − 1 nT/ R E were associated with transient dipolarizations, propagating Earthward. There was no clear association of local plasma sheet activity onset with any value of ∂  B z /∂  x . We discuss relation of observations and recent modeling results.

Description: [1]  Large explosive volcanic eruptions can generate ash clouds from rising plumes that spread in the atmosphere around a Neutral Buoyancy Level (NBL). These ash clouds spread as inertial intrusions and are advected by atmospheric winds. For low mass flow rates, tephra transport is mainly dictated by wind advection, because ash cloud spreading due to gravity current effects is negligible (passive transport). For large mass flow rates, gravity driven transport at the NBL can be the dominant transport mechanism. Conditions under which the passive transport assumption is valid have not yet been critically studied. We analyze the conditions when gravity-driven transport is dominant in terms of the cloud Richardson number. Moreover, we couple an analytical model that describes cloud spreading as a gravity current with an advection–diffusion model. This coupled model is used to simulate the evolution of the volcanic cloud during the climatic phase of the 1991 Pinatubo eruption.

Description: [1]  Regimes of tropical low-level clouds are commonly identified according to large-scale subsidence and lower tropospheric stability (LTS). This definition alone is insufficient for the distinction between regimes and limits the comparison of low-level clouds from CloudSat radar observations and the ECHAM5 GCM run with the COSP radar simulator. Comparisons of CloudSat radar cloud altitude-reflectivity histograms for stratocumulus and shallow cumulus regimes, as defined above, show nearly identical reflectivity profiles because the distinction between the two regimes is dependent upon atmospheric stability below 700 hPa and observations above 1.5 km. Regional subsets, near California and Hawaii for example, have large differences in reflectivityprofiles than the dynamically defined domain; indicating different reflectivity profiles exist under a given large-scale environment. Regional subsets are better for the evaluation of low-level clouds in CloudSat and ECHAM5 as there is less contamination between 2.5 km to 7.5 km from precipitating hydrometeors which obscured cloud reflectivities.

Description: [1]  Deep moist atmospheric convection is a key element of the weather and climate system for transporting mass, momentum, and thermal energy. It has been challenging to simulate convection realistically in global atmospheric models, because of the large gap in spatial scales between convection (10 0  km) and global motions (10 4  km). We conducted the first ever sub-kilometer global simulation and described the features of convection. Through a series of grid-refinement resolution testing, we found that an essential change for convection statistics occurred around 2-km grid spacing. The convection structure, number of convective cells, and distance to the nearest convective cell dramatically changed at this resolution. The convection core was resolved using multiple grids in simulations with grid spacings less than 2.0 km.

Description: [1]  Dry conditions from a moderate El Nino during the fall of 2006 resulted in enhanced burning in Indonesia with fire emissions of CO approximately 4–6 times larger than the prior year. Here we use new tropospheric methane and CO data from the Aura Tropospheric Emission Spectrometer (TES) and new CO profile measurements from the Terra Measurements of Pollution in the Troposphere (MOPITT) satellite instruments with the GEOS-Chem model to estimate methane emissions of 4.25 +/− 0.75 Tg for October-November 2006 from these fires. Errors in convective parameterization in GEOS-Chem, evaluated by comparing MOPITT and GEOS-Chem CO profiles, are the primary uncertainty of the emissions estimate. The El Nino related Indonesian fires increased the tropical distribution of atmospheric methane relative to 2005, indicating that tropical biomass burning can compensate for expected decreases in tropical wetland methane emissions from reduced rainfall during El Nino as found in previous studies.

Description: [1]  In this study, we investigate the formation predictability of Hurricane Sandy (2012) with a global mesoscale model. We first present five track and intensity forecasts of Sandy initialized at 00Z October 22-26, 2012, realistically producing its movement with a northwestward turn prior to its landfall. We then show that three experiments initialized at 00Z Oct. 16-18 captured the genesis of Sandy with a lead time of up to six days and simulated reasonable evolution of Sandy's track and intensity in the next two-day period of 18Z Oct. 21-23. Results suggest that the extended lead time of formation prediction is achieved by realistic simulations of multi-scale processes, including (1) the interaction between an easterly wave and a low-level westerly wind belt (WWB); (2) the appearance of the upper-level trough at 200-hPa to Sandy's northwest. The low-level WWB and upper-level trough are likely associated with a Madden-Julian Oscillation.

Description: [1]  Water vapor is an important greenhouse gas in the earth's atmosphere. Absorption of the solar radiation by water vapor in the near UV region may partially account for the up to 30% discrepancy between the modeled and the observed solar energy absorbed by the atmosphere. But the magnitude of water vapor absorption in the near UV region at wavelengths shorter than 384 nm is not known. We have determined absorption cross sections of water vapor at 5 nm intervals in the 290-350 nm region, by using cavity ring-down spectroscopy. Water vapor cross section values range from 2.94 × 10 -24 to 2.13 × 10 -25  cm 2 /molecule in the wavelength region studied. The effect of the water vapor absorption in the 290-350 nm region on the modeledradiation flux at the ground level has been evaluated using radiative transfer model.

Description: [1]  The RAPID-MOCHA array has observed the Atlantic Meridional overturning circulation (AMOC) at 26.5°N since 2004. During 2009/2010, there was a transient 30% weakening of the AMOC driven by anomalies in geostrophic and Ekman transports. Here, we use simulations based on the Met Office Forecast Ocean Assimilation Model (FOAM) to diagnose the relative importance of atmospheric forcings and internal ocean dynamics in driving the anomalous geostrophic circulation of 2009/10. Data assimilating experiments with FOAM accurately reproduce the mean strength and depth of the AMOC at 26.5°N. In addition, agreement between simulated and observed stream functions in the deep ocean is improved when we calculate the AMOC using a method that approximates the RAPID observations. The main features of the geostrophic circulation anomaly are captured by an ensemble of simulations without data-assimilation. These model results suggest that the atmosphere played a dominant role in driving recent interannual variability of the AMOC.

Description: [1]  Understanding and explaining the trend in GMSL has important implications for future projections of sea level rise. While measurements from satellite altimetry have provided accurate estimates of GMSL, the modern altimetry record has only now reached twenty years in length, making it difficult to assess the contribution of decadal to multi-decadal climate signals to the global trend. Here, we use a sea level reconstruction to study the twenty-year trends in sea level since 1950. In particular, we show that the Pacific Decadal Oscillation (PDO) contributes significantly to the twenty-year trends in GMSL. We estimate the PDO contribution to the GMSL trend over the past twenty years to be approximately 0.49 ± 0.25 mm/year, and find that removing the PDO contribution reduces the acceleration in GMSL estimated over the past sixty years.

Description: [1]  This study examines links between the North Pacific Gyre Oscillation (NPGO) and the occurrence of tropical cyclones (TCs) in the western North Pacific (WNP) at both inter-annual and decadal scales. The major findings are summarized as follows: (1) NPGO makes significant impacts on the WNP TC frequency at both inter-annual and decadal time scales. The impacts of NPGO on the TC activity are more profound than those exerted by the Pacific Decadal Oscillation (PDO) and the El Niño Southern Oscillation. (2) Niño 3 plays a more important role than Niño 3.4 and Niño 4 in modulating the decadal WNP TC activity. (3) Positive low-level relative vorticity and weak zonal vertical wind shear are responsible for the increase in the WNP TC activity in negative NPGO phases. This study indicates that the NPGO and Niño 3 indices should be key factors for building a scheme for decadal prediction of occurrences of WNP TCs.

Description: [1]  ELF/VLF radio waves are difficult to generate with conventional antennas. Ionospheric HF heating facilities generate ELF/VLF waves via modulated heating of the lower ionosphere. HF heating of the ionosphere changes the lower ionospheric conductivity, which in the presence of natural currents such as the auroral electrojet, creates an antenna in the sky when heating is modulated at ELF/VLF frequencies. We present a summary of nearly 100 days of ELF/VLF wave generation experiments at the 3.6 MW HAARP facility near Gakona, Alaska, and provide a baseline reference of ELF/VLF generation capabilities with HF heating. Between February 2007 and August 2008, HAARP was operated on close to 100 days for ELF/VLF wave generation experiments, at a variety of ELF/VLF frequencies, seasons and times of day. We present comprehensive statistics of generated ELF/VLF magnetic fields observed at a nearby site, in the 500-3500 Hz band. Transmissions with a specific HF beam configuration (3.25 MHz, vertical beam, amplitude modulation) are isolated so the data comparison is self-consistent, across nearly 5 million individual measurements of either a tone or a piece of a frequency-time ramp. There is a minimum in the average generation close to local midnight. It is found that generation during local nighttime is on average weaker, but more highly variable, with a small number of very strong generation periods. Signal amplitudes from day to day may vary by as much as 20-30 dB. Generation strengthens by ~5 dB during the first ~30 minutes of transmission, which may be a signature of slow electron density changes from sustained HF heating. Theoretical calculations are made to relate the amplitude observed to the power injected into the waveguide and reaching250 km. The median power generated by HAARP and injected into the waveguide is ~0.05-0.1 W in this base-line configuration (vertical beam, 3.25 MHz, amplitude modulation), but may have generated hundreds of Watts for brief durations. Several efficiency improvements have improved the ELF/VLF wave generation efficiency further.

Description: [1]  An inversion technique for estimating the properties of the magnetospheric plasma from the harmonic frequencies of the toroidal standing Alfvén waves has been used to derive the global equatorial mass density covering radial distances from 4 to 9 Earth radii ( R E ), within the local time sector spanning from 0300 to 1900 hours. This broad range of L shell extending to the outer magnetosphere allows us to examine the local time and radial dependence of the quiet-time equatorial mass density during solar minimum and thereby construct a global distribution of the equatorial mass density. The toroidal Alfvén waves were detected with magnetometers on the Active Magnetospheric Particle Tracer Explorers (AMPTE)/Charge Composition Explorer (CCE) during the nearly 5 year interval from August 1984 to January 1989 and on the Geostationary Operational Environmental Satellites (GOES) (10, 11 and 12) for 2 years from 2007 to 2008, both of which were operating during solar minimum years. The derived equatorial mass density, ρ eq , at geosynchronous orbit (GEO) monotonically increases with increasing magnetic local time (MLT) from the nightside towards the dusk sector. At other radial distances, ρ eq has the same MLT variation as that of GEO, while the magnitude logarithmically decreases with increasing L value. An investigation of the Dst and Kp dependence shows that the median value of ρ eq varies little in the daytime sector during moderately disturbed times, which agrees with previous studies. ρ eq calculated from the F 10.7 dependent empirical model shows good agreement with that of CCE but overestimates that of GOES probably due to the extreme solar cycle minimum in years 2007–2008.

Description: [1]  In this paper we compare observations of the high latitude cusp from DMSP data to simulations conducted using the Lyon-Fedder-Mobarry (LFM) global magnetosphere simulation. The LFM simulation is run for the 31 Aug 2005 to 02 Sep 2005 moderate storm, from which the solar wind data exhibits a wide range of conditions that enable a statistical representation of the cusp to be obtained. The location of the cusp is identified using traditional magnetic depression and plasma density enhancement at high altitude. A new diagnostic using the parallel ion number flux is also tested for cusp identification. The correlation of the cusp latitude and various solar wind IMF coupling functions is explored using the three different cusp identification methods. The analysis shows 1) the three methods give approximately the same location and size of the simulated cusp at high altitude; 2) the variations of the simulated cusp are remarkably consistent with the observed statistical variations of the low-altitude cusp. In agreement with observations a higher correlation is obtained using other solar wind coupling functions such as the Kan-Lee electric field. The MLT position of the simulated cusp is found to depend upon the IMF By component, with a lower linear correlation. The width of the simulated cusp in both latitude and MLT is also examined. The size of the cusp is found to increase with the solar wind dynamic pressure with saturation seen when the dynamic pressure is greater than 3 nPa.

Description: [1]  Inter-plate coupling on the Hikurangi subduction margin along the east coast of New Zealand's North Island changes north to south from almost uncoupled to locked. Clay-rich sediments and aqueous-fluids at the subduction interface have been invoked as key factors in the frictional processes that control inter-plate coupling. Here we use magnetotelluric (MT) data to show that the subduction interface in the weakly coupled region is electrically conductive, but is resistive in the locked region. These results indicate the presence of a layer of fluid- and clay-rich sediments in the weakly coupled region and support the idea that the presence of fluid and hydrated clays at the interface is a major factor controlling plate coupling.

Description: [1]  We report a new type of variations in Doppler velocity of HF ground scatter echoes from the polar cap at f ~10 mHz. Similar fluctuations from lower latitudes are usually associated with large-scale dayside Pc3-4 ULF waves. However, the polar cap oscillations exhibit a puzzling anisotropy in spatial coherence along and across the radar's line-of-sight. Furthermore, in contrast to Pc3-4 waves, these fluctuations show no ground magnetic signatures and display a pronounced gap in power/occurrence around local noon. We hypothesize that localized, ≤100 km, auroral particle precipitations near the radar site can modulate Doppler shift of the radio waves entering the ionosphere. In the ground scatter returns, due to the geometrical spread of the rays propagating through the ionosphere to the ground, these variations would appear to have a much larger line-of-sight scale.

Description: [1]  Daily profiles of phase measurements as observed on fixed VLF-paths generally show a transient phase advance, followed by a phase delay, for about 90 minutes after sunrise hours. This is indicative of a reflecting ionospheric C-region developing along the terminator line at an altitude below the normal D-region. The suggested occurrence of a C-region is consistent with rocket measurements made in the 1960's, showing a maximum of the electron density between 64 and 68 km, and by radio sounding in the 1980's. In order to correctly describe the properties of the phase effect associated with the presence of a C-region, it is important to understand the subionospheric propagation characteristics of the VLF-paths. In this paper, we analyze the variations presented by the temporal properties of the VLF narrow-band phase effect, and determined a parameter associated with the appearance of the C-region at sunrise hours observed by receivers from the South America VLF Network (SAVNET). Periodic patterns emerge from the parameter curves. Two distinct temporal behavior regimes can be identified: one exhibiting slow variations between March and October, and another one exhibiting faster variations between October and March. Solar illumination conditions and the geometrical configuration of the VLF paths relative to the sunrise terminator partly explain the slow variation regime. During periods of faster variations, we have observed good association with atmospheric temperature variability found in the measurements of the TIMED-SABER satellite instrument, which we assume to be related to the Winter Anomaly atmospheric phenomenon. However, when comparing the parameter time series with temperature curves, no direct one-to-one correspondence was found for transient events.

Description: [1]  On 21 January 2005, one of the fastest interplanetary coronal mass ejections (ICME) of solar cycle 23, containing exceptionally dense plasma directly behind the sheath, hit the magnetosphere. We show from charge-state analysis that this material was a piece of the erupting solar filament, and further, based on comparisons to the simulation of a fast CME, that the unusual location of the filament material was a consequence of three processes. As the ICME decelerated, the momentum of the dense filament material caused it to push through the flux rope towards the nose. Diverging non-radial flows in front of the filament moved magnetic flux to the sides of the ICME. At the same time reconnection between the leading edge of the ICME and the sheath magnetic fields worked to peel away the outer layers of the flux rope creating a remnant flux rope and a trailing region of newly opened magnetic field lines. These processes combined to move the filament material into direct contact with the ICME sheath region. Within one hour after impact and under northward IMF conditions, a cold dense plasma sheet formed within the magnetosphere from the filament material. Dense plasma sheet material continued to move through the magnetosphere for more than 6 hours as the filament passed by the Earth. Densities were high enough to produce strong diamagnetic stretching of the magnetotail despite the northward IMF conditions and low levels of magnetic activity. The disruptions from the filament collision are linked to an array of unusual features throughout the magnetosphere, ionosphere and atmosphere. These results raise questions about whether rare collisions with solar filaments may, under the right conditions, be a factor in producing even more extreme events.

Description: [1]  We present observations from the Falkland Islands SuperDARN radar of the propagation of HF radio waves via the Weddell Sea ionospheric Anomaly (WSA), a region of enhanced austral summer nighttime ionospheric electron densities covering the southern Pacific and South Americas region. This anomaly is thought to be produced by uplift of the ionosphere by prevailing equatorwards thermospheric winds. Of particular interest are perturbations of the WSA-supported propagation, which suggest that during periods of geomagnetic disturbance the ionospheric layer can be lowered by several 10s of km and subsequently recover over a period of 1 to 2 hours. Perturbations can appear singly or as a train of 2 to 3 events. We discuss possible causes of the perturbations, and conclude that they are associated with equatorward-propagating large-scale atmospheric waves produced by magnetospheric energy deposition in the auroral or sub-auroral ionosphere. Changes in high/mid-latitude electrodynamics during geomagnetic storms may also account for the perturbations, but further modeling is required to fully understand their cause.

Description: [1]  In this study we present 3D data assimilation using CRRES data and 3D Versatile Electron Radiation Belt Model using a newly developed operator-splitting method. Simulations with synthetic data show that the operator-splitting Kalman filtering technique proposed in this study can successfully reconstruct the underlying dynamic evolution of the radiation belts. The method is further verified by the comparison with the conventional Kalman filter. We applied the new approach to 3D data assimilation of real data, to globally reconstruct the dynamics of the radiation belts using pitch-angle, energy, and L-shell dependent CRRES observations. An L-shell time cross-sections of the global data assimilation results for nearly equatorially mirroring particles and high and low values of the first adiabatic invariants clearly show the difference between the radial profiles of phase space density. At μ  = 700 MeV/G cross-section of the global reanalysis shows a clear peak in the phase space density, while at lower energy of 70 MeV/G the profiles are monotonic. Since the radial profiles are obtained from one global reanalysis, the differences in the profiles reflect the differences in the underlying physical processes responsible for the dynamic evolution of the radiation belt energetic and relativistic electrons.

Description: [1]  Inner magnetosphere magnetic field and plasma flow data are examined during 228 steady magnetospheric convection events. We find that the B Z component of the magnetic field around geostationary orbit is weaker than during average conditions and the plasma flow speeds are higher than average in the dusk sector just beyond geostationary orbit. The SMC periods include more enhanced Earthward and tailward flow intervals than during average conditions. The steady convection period magnetic field is not steady: The near-geostationary nightside field grows increasingly taillike throughout the steady convection period. In the midtail, Earthward flows are enhanced in a wide region around the midnight sector, which leads to enhanced magnetic flux transport toward the Earth during the steady convection periods. Compared to well-known characteristics during magnetospheric substorms, the inner tail evolution resembles that during the substorm growth phase, while the midtail flow characteristics duringsteady convection periods are similar to those found during substorm recovery phases.

Description: [1]  This study evaluates the potential of a proposed technique in using satellite-borne radiometer measurements and weather analyses to estimate the intensity of tropical cyclones. This theory shows that intensity is essentially directly related to the temperature deficit of cloud-top versus sea-surface, and the surplus in saturation entropy in the eyewall versus its surroundings. The eyewall entropy estimate comes from measurements of cloud-top temperature and pressure, and the analysis provides the environmental saturation entropy. An Observing Systems Simulation Experiment (OSSE) was conducted and the results were compared to those from previous studies using cloud-profiling radar altimetry measurements. The use of cloud-top pressure measurements may produce more accurate results. Inherent challenges still require caution in considering operational implementation.

Description: [1]  A global total electron content (TEC) model response to geomagnetic activity described by the K p -index is built by using the Center for Orbit Determination of Europe (CODE) TEC data for full 13 years, January 1999 - December 2011. The model describes the most probable spatial distribution and temporal variability of the geomagnetically forced TEC anomalies assuming that these anomalies at a given modip latitude depend mainly on the K p -index, local time (LT) and longitude. The geomagnetic anomalies are expressed by the relative deviation of TEC from its 15-day median and are denoted as rTEC. The rTEC response to the geomagnetic activity is presented by a sum of two responses with different time delay constants and different sign of the cross-correlation function. It has been found that the mean dependence of rTEC on K p -index can be expressed by a cubic function. The LT dependence of rTEC is described by Fourier time series which includes the contribution of four diurnal components with periods 24, 12, 8 and 6 hours. The rTEC dependence on longitude is presented by Fourier series which includes the contribution of zonal waves with zonal wavenumbers up to 6. In order to demonstrate how the model is able to reproduce the rTEC response to geomagnetic activity three geomagnetic storms at different seasons and solar activity conditions are presented. The model residuals clearly reveal two types of the model deviation from the data: some underestimation of the largest TEC response to the geomagnetic activity and randomly distributed errors which are the data noise or anomalies generated by other sources. The presented TEC model fits to the CODE TEC input data with small negative bias of -0.204, root mean squares error RMSE  = 4.592 and standard deviation error STDE  = 4.588. The model offers TEC maps which depend on geographic coordinates (5 o x5 o in latitude and longitude), and universal time (UT) at given geomagnetic activity and day of the year. It could be used for both science and possible service (nowcasting and short-term prediction); for the latter a detailed validation of the model at different geophysical conditions has to be performed in order to clarify the model predicting quality.

Description: [1]  Magnetic reconnection (MR), a fundamental process in space plasmas that changes magnetic topology and converts magnetic energy into kinetic and thermal energies, is an ultimate driver of space weather. There exist two models of MR in the literatures, anti-parallel and component, associated with intensive studies of their generation and applications. In this paper we report an MR event observed by Cluster constellation in the geo-magnetotail where both types of MR were detected. By reconstructing the three-dimensional (3D) MR configuration we find that a pair of A-B nulls existed in both types of MR cases with two fan surfaces intersecting each other to form a separator line connecting the nulls. A weak or sizable magnetic field exists along the separator in the anti-parallel or component case, respectively. In the latter case, field strength is finite away from the two nulls and vanishes close to the nulls. Therefore, at least in the two cases observed, both anti-parallel and component MR geometries are local presentation of the separator MR configuration. This result supports the expectation that 3D nulls often occur as a crucial element of MR at least in the magnetotail and separator MR may play an important role in dynamics and reconfiguration of magnetic field in 3D MR processes.

Description: [1]  Structural evolution of monsoon clouds in the core monsoon region of India has been examined using multi-sensor data. Invigoration of warm clouds above 4.5 km (occurring in only 15.4% days of the last 11 monsoon seasons) is associated with a transition from negative to positive normalized rainfall anomaly. Cloud top pressure reduces with an increase in aerosol optical depth at a higher rate of invigoration in drier condition (characterized by large fraction of absorbing aerosols) than wet condition. Cloud effective radius for warm clouds does not show any significant change with an increase in aerosol concentration in presence of high liquid water path, probably due to strong buffering role of meteorology. The structural evolution of monsoon clouds is influenced by both dynamic and microphysical processes that prolong the cloud lifetime, resulting in infrequent rainfall. Our results call for improved representation of aerosol and cloud vertical structures in the climate models to resolve this issue.

Description: [1]  The development of equatorial plasma irregularity plumes can be well recorded by steerable backscatter radars operated at and off the magnetic equator due to the fact that the vertically extended plume structures are tracers of magnetically north-south aligned larger scale structures. From observations during March 2012, using two low latitude steerable backscatter radars in Southeast Asia, the Equatorial Atmosphere Radar (EAR) (0.2ºS, 100.3ºE; dip lat 10.4ºS) and the Sanya VHF radar (18.4ºN, 109.6ºE; dip lat 12.8ºN), the characteristics of backscatter plumes over the two sites separated in longitude by ~1000 km were simultaneously investigated. The beam steering measurements reveal frequent occurrences of multiple plumes over both radar sites, of which two cases are analyzed here. The observations on 30 March 2012 show plume structures initiated within the radar scanned area, followed by others drifting from the west of the radar beam over both stations. A tracing analysis on the onset locations of plasma plumes reveals spatially well-separated backscatter plumes, with a maximum east-west wavelength of about 1000 km, periodically generated in longitudes between 85ºE and 110ºE. The post-sunset backscatter plumes seen by the Sanya VHF radar are found to be due to the passage of sunset plumes initiated around the longitude of EAR. Most interestingly, the EAR measurements on the night of 21 March 2012 show multiple plume structures that developed successively in the radar scanned area with east-west separation of ~50 km, with however, no sunset plasma plume over Sanya. Co-located ionogram measurements show that spread F irregularities occurred mainly in the bottomside F-region at Sanya, whereas satellite traces in ionograms that are indications of large-scale wave structures, were observed on that night at both stations. Possible causes for the longitudinal difference in the characteristics of radar backscatter plumes are discussed.

Description: [1]  An increased frequency and intensity of winter and spring storms have recently manifested over a broad area of North America—along the east coast of the U.S. especially, though global mean storm tracks are suggested to shift northward. To understand these changes, we have conducted atmospheric model experiments, examining the response of North American storm activity to the elevated tropical Pacific sea surface temperature (SST) associated with El Niño. The results indicate that, when tropical Pacific SST increases, there are more numerous intense storms over southwestern, southeastern, and northwestern North America, but fewer weaker storms over the northeast. Transient eddy analysis of the general circulation demonstrates consistent changes, suggesting systematic changes from large-scale general circulation to synoptic-scale storms. These changes can be attributed to enhanced lower tropospheric baroclinicity, to which the southward shift and an intensification of extratropical jet streams make a major contribution.

Description: [1]  Methane is a potent greenhouse gas and large-scale rapid release of methane from hydrate may have contributed to past abrupt climate change inferred from the geological record. The discovery in 2008 of over 250 plumes of methane gas escaping from the seabed of the West Svalbard continental margin at ~400 m water depth (mwd), suggests that hydrate is dissociating in the present-day Arctic. Here, we model the dynamic response of hydrate-bearing sediments over a period of 2300 yr and investigate ocean warming as a possible cause for present-day and likely future dissociation of hydrate, within 350-800 mwd, west of Svalbard. Future temperatures are given by two climate models, HadGEM2 and CCSM4, and scenarios, Representative Concentration Pathways (RCPs) 8.5 and 2.6. Our results suggest that over the next three centuries 5.3-29 Gg yr -1 of methane may be released to the Arctic Ocean on the West Svalbard margin.

Description: [1]  In recent years it has been discovered that sections of the subduction interface slip aseismically in slow slip events, during which stress is intermittently transferred to the section of the subduction zone that generates large or great earthquakes. Within the Cascadia subduction zone the magnitude and frequency of SSEs and accompanying tectonic tremor exhibit complex patterns that vary systematically with depth. However the loading mechanisms and interactions that precede great subduction earthquakes are poorly understood. Here we present results from physics-based simulations that reproduce the continuum of SSE characteristics reported for the Cascadia subduction zone. The simulations provide a basis for understanding the interactions that control both the observed complex patterns of SSEs and stress transfer to the seismogenic section that produce great earthquakes.

Description: [1]  Clouds over the Southern Ocean exist in a pristine environment that results in unique microphysical properties. However, in-situ observations of these clouds are rare, and the dominant precipitation processes are unknown. Uncertainties in their life cycles and radiative properties make them interesting from a weather and climate perspective. Data from the standard cloud physics payload during the HIAPER Pole-to-Pole Observations (HIPPO) global transects provide a unique snapshot the nature of low-level clouds in the Southern Ocean. High quantities of supercooled liquid water (up to 0.47gm –3 ) were observed in clouds as cold as –22 °C during two flights in different seasons and different meteorological conditions, supporting climatologies inferred from satellite observations. Cloud droplet concentrations were calculated from mean droplet size and liquid water concentrations, and were in the range of 30–120 cm –3 , which is fairly typical for the pristine Southern Ocean environment. Ice in non- or lightly-precipitating clouds was found to be rare, while drizzle drops with diameter greater than 100  μ m formed through warm rain processes were widespread. Large, pristine crystals were commonly seen in very low concentrations below cloud base.

Description: [1]  The India-Eurasia collision is responsible for producing the Himalayan Mountains and Tibetan plateau and has been hypothesized to have significant far field influences, including driving the Baikal rift and the eastward extrusion of South China. However, quantification of lithospheric buoyancy forces and integrated effect of tractions acting at base of the lithosphere are unable to explain the observed surface motions within South China. We present 198 new SKS shear-wave splitting observations beneath South China and invert these data along with published GPS data to solve for the sub-asthenospheric flow field beneath South China to assess the role of small-scale convection here. We find a 15-20 mm/yr southwestward-directed mantle flow towards the Burma slab. This flow is consistent with the mantle response of slab retreat over the past 25ma, and counter flow due to subduction of Burma/Sunda slabs demonstrating the importance of localized mantle convection on present day plate motions.

Description: [1]  The solar wind impacting the Earth varies over a wide range of time scales, driving a corresponding range of geomagnetic activity. Past work has strongly indicated the rate of merging on the frontside magnetosphere is the most important predictor for magnetospheric activity, especially over a few hours. However the magnetosphere exhibits variations on other time scales, including UT, seasonal, and solar cycle variations. Much of this geomagnetic variation cannot be reasonably attributed to changes in the solar wind driving – that is, it is not created by the original Russell-McPherron effect or any generalization thereof. In this paper we examine the solar cycle, seasonal, and diurnal effects based upon the frequency of substorm onsets, using a data set of 53,000 substorm onsets. These were identified through the SuperMAG collaboration and span three decades with continuous coverage. Solar cycle variations include a profound minima in 2009 (448 substorms) and peak in 2003 (3727). The magnitude of this variation (a factor of 8.3) is not explained through variations in estimators of the frontside merging rate (such as d Φ MP / dt ), even when the more detailed probability distribution functions are examined. Instead, v , or better, n 1/2 v 2 seems to be implicated in the dramatic difference between active and quiet years, even beyond the role of velocity in modulating merging. Moreover, we find that although most substorms are preceded by flux loading (78.5% are above the mean and 83.8% above median solar wind driving) a high solar wind v is almost as important (68.3% above mean, 74.8% above median). This and other evidence suggests either v or n 1/2 v 2 (but probably not p ) plays a strong secondary role in substorm onset. As for the seasonal and diurnal effects, the elliptical nature of the Earth's orbit, which is closest to the Sun in January, leads to a larger solar wind driving (measured by Bs , vBs , or d Φ MP / dt ) in November, as is confirmed by 22 years of solar wind observations. However substorms peak in October and March, and have a UT dependence best explained by whether a conducting path established by solar illumination exists in at least one hemisphere in the region where substorm onsets typically occur.

Description: [1]  The primary sources of energetic electron precipitation (EEP) which affect altitudes 〈100 km (〉30 keV) are expected to be from the radiation belts, and during substorms. EEP from the radiation belts should be restricted to locations between L  = 1.5-8, while substorm produced EEP is expected to range from L  = 4-9.5 during quiet geomagnetic conditions. Therefore, one would not expect any significant D-region impact due to electron precipitation at geomagnetic latitudes beyond about L  = 10. In this study we report on large unexpectedly high latitude D-region ionization enhancements, detected by an incoherent scatter radar at L  ≈ 16, which appear to be caused by electron precipitation from substorms. We go on to reexamine the latitudinal limits of substorm produced EEP using data from multiple low-Earth orbiting spacecraft, and demonstrate that the precipitation stretches many hundreds of kilometers polewards of the previously suggested limits. We find that a typical substorm will produce significant EEP over the IGRF L -shell range L  = 4.6 ± 0.2-14.5 ± 1.2, peaking at L  = 6-7. However, there is significant variability from event to event; in contrast to the median case, the strongest 25% of substorms have significant EEP in the range spaning L  = 4.1 ± 0.1-20.7 ± 2.2, while the weakest 25% of substorms have significant EEP in the range spaning L  = 5.5 ± 0.1-10.1 ± 0.7. We also examine the occurrence probability of very large substorms, focusing on those events which appear to be able to disable geostationary satellites when they are located near midnight MLT. On average these large substorms occur approximately 1-6 times per year, a significant rate given the potential impact on satellites.

Description: [1]  This paper presents new observations of the behaviour of simulated dust particles in space plasma based on a 3D particle in cell code. Multistep Monte Carlo collision is employed to simulate the dust charging process which is validated for the cases of charging of isolated dust particle and ensemble dust particles, where results indicate good agreement between simulation and theories. The code is then used to investigate plasma properties near a charged surface in a vicinity of a cloud of dust particles. The simulation reveals that a cloud of dust particle close to a spacecraft surface affects plasma densities around the spacecraft as well the spacecraft's surface potential. It is suggested that dust cloud causes the surface to charge to higher negative potential. The simulation also suggests that the combination of surface potential and dust cloud potential produces a region of trapped low energy electrons.

Description: [1]  Rotational along with translational and strain measurements are essential for a complete description of the motion of a deformable body in a seismic event. We propose a new seismogeodetic approach where collocated high-rate GPS and accelerometer measurements are combined to estimate permanent and dynamic coseismic ground tilts at a point, whereas at present only dynamic tilts are measured with either a dense seismic array or an expensive ring laser gyroscope. We estimate point tilts for a five-story structure on a shake table subjected to 13 earthquake strong-motion records of increasing intensity. For the most intense record from the 2002 M7.9 Denali earthquake, we observe a peak-to-peak dynamic tilt of 0.12°, and a permanent tilt of 0.16° for the structure's roof. Point tilts derived from networks of collocated GPS and accelerometers can be used to estimate the rotational component of seismic wavefield for improved earthquake source characterization.

Description: [1]  Following the recent discovery of the “Modoki” El Niño, a proliferation of studies and debates has ensued concerning whether Modoki is dynamically distinct from “Canonical” El Niño, how Modoki impacts and teleconnections differ, and whether Modoki events have been increasing in frequency or amplitude. Three decades of reliable, high temporal–resolution observations of coupled ocean–atmosphere variability in the equatorial Pacific reveal a rich diversity of El Niños. Although central and eastern Pacific sea surface temperature (SST) anomalies appear mechanistically separable in terms of local and remote forcing, their frequent overlap precludes robust classifications. All observed El Niños appear to be a mixture of locally (central Pacific) and remotely forced (eastern Pacific) SST anomalies. Submonthly resolution appears essential for this insight and for the proper dynamical diagnosis of El Niño evolution, thus the use of long–term monthly reconstructions for classification and trend analysis is strongly cautioned against.

Description: [1]  Sea surface salinity (SSS) measurements from the Aquarius/SAC-D satellite and SMOS mission were used to document the freshening associated with the record 2011 Mississippi River flooding event in the Gulf of Mexico (GoM). Assessment of the salinity response was aided by additional satellite observations, including MODIS chlorophyll-a (chl-a) and ocean surface currents, and a passive tracer simulation. Low SSS values associated with the spreading of the river plume were observed 1–3 months after peak river discharge which then receded and became unidentifiable from satellite observations 5 months after maximum discharge. The seasonal wind pattern and general circulation of the GoM dramatically impacted the observed salinity response, transporting freshwater eastward along the Gulf coast and entraining low salinity waters into the open GoM. The observed salinity response from Aquarius was consistent with SMOS SSS, chl-a concentrations, and the passive tracer simulation in terms of the pathway and transit time of the river plume spreading. This study is the first successful application of satellite SSS to study salinity variation in marginal seas.

Description: [1]  This study uses a simulation method to explore how estuarine pH is affected by mixing between river water, anthropogenic CO 2 enriched seawater, and by respiration. Three rivers with different levels of weathering products (Amazon, Mississippi, and St. Johns) are selected for this simulation. The results indicate that estuaries that receive low to moderate levels of weathering products (Amazon and St. Johns) exhibit a maximum pH decrease in the mid-salinity region as a result of anthropogenic CO 2 intrusion. This maximum pH decrease coincides with a previously unrecognized mid-salinity minimum buffer zone (MBZ). In addition, water column oxygen consumption can further depress pH for all simulated estuaries. We suggest that recognition of the estuarine MBZs may be important for studying estuarine calcifying organisms and pH-sensitive biogeochemical processes.

Description: [1]  A simple wavy magnetodisk model can explain periodicities in energetic charged particles observed in Saturn's outer magnetosphere (〉20 R S ). The model's free parameters are the tilt of magnetodisk in inner magnetosphere (~1.8°), speed of outgoing spiral wave (~8 R S /hr), critical radius (~10 R S ), and period of rotation (10.64 hours). The fidelity of the model is not judged by a least squares fit to the actual data, but rather by the model's accuracy in reproducing the Lomb periodogram of the periodicities. The model accurately simulates the main spectral feature near ~10.7 hours plus a secondary (“dual”) period near ~10.95 hours. The ability of the wavy magnetodisk with one period to produce the observed dual periodicity in the observations suggests that models having “dual” periods need not be invoked to explain some of the periodicities in Saturn's outer magnetosphere.

Description: [1]  The fluid-filled crack model has been widely used to interpret the peak frequencies of long-period (LP) seismic events at volcanoes. Up to now, numerical methods have been used to compute resonant frequencies in the model. We propose a simple analytical formula for the longitudinal resonance frequencies of a fluid-filled crack. We evaluated the formula by comparing its results with the resonant frequencies computed by finite difference method (FDM) code. The comparison revealed that the formula well describes the resonant frequencies of both 2D and 3D cracks. The formula enables simple and rapid estimates of the fluid properties and geometries of LP source cracks.

Description: [1]  We determined the compressional velocity of hcp-Fe using high resolution inelastic X-ray scattering (IXS) combined with in-situ X-ray powder diffraction (XRD): our measurements extend up to 174 GPa at room temperature, to 88 GPa at 700 K, and to 61.5 GPa at 1000 K. Our data, including those obtained at high temperature, are well described by a linear relation to density, extending the range of verification of Birch's law, and suggesting only small temperature dependence up to 1000 K. This result, once compared to the PREM seismologically based model, indicates that there is either a strong temperature effect on Birch's law above 1000 K, or that the composition of the core is rather different than expected, containing, e.g. heavy impurities. Noting that both recent theoretical calculations and shock-wave velocity measurements are consistent with modification of Birch's law at high temperature, we favor the former explanation.

Description: [1]  We here document with magnetic field observations a passage of the MESSENGER spacecraft through Mercury's magnetosphere under conditions of a quasi-parallel bow shock, i.e., when the direction of the upstream interplanetary magnetic field (IMF) was within 45° of the bow shock normal. The spacecraft's fast transition of the magnetosheath and the steady solar wind conditions during the period analyzed allow both spatial and temporal properties of the shock crossing to be investigated. The observations show that the shock reformation process can be nearly periodic under stable solar wind conditions. Throughout the 25-min-long observation period, the pulsation duration deviated by at most ~10% from the average 10 s period measured. This quasi-periodicity allows us to study all aspects of the shock reconfiguration, including ultra-low-frequency waves in the upstream region and large-amplitude magnetic structures observed in the vicinity of the magnetosheath − solar wind transition region and inside the magnetosheath. We also show that bow shock reformation can be a substantial source of wave activity in the magnetosphere, on this occasion having given rise to oscillations in the magnetic field with peak-to-peak amplitudes of 40–50 nT over large parts of the dayside magnetosphere. The clean and cyclic behavior observed throughout the magnetosphere, the magnetosheath, and the upstream region indicates that the subsolar region was primarily influenced by a cyclic reformation of the shock front, rather than by a spatial and temporal patchwork of short large-amplitude magnetic structures (SLAMS), as is generally the case at the terrestrial bow shock under quasi-parallel conditions.

Description: [1]  The entry of solar wind into the magnetosphere is strongly influenced by kinetic-scale boundary layers where the rapid variation in the magnetic field and/or velocity can drive transport. In current layers with strong Alfvénic velocity shear, the generation of vortices from the Kelvin-Helmholtz instability can drive magnetic reconnection even in broader current sheets by locally compressing these layers as the vortices develop. Previous two-dimensional (2D) fully kinetic simulations of this vortex-induced reconnection process have demonstrated the copious formation of magnetic islands in regions of strongly compressed current between the vortices. Here we describe the first three-dimensional (3D) fully kinetic simulations of this process and demonstrate that the compressed current sheets give rise to magnetic flux ropes over a range of oblique angles and along the entire extent of the compressed current layer around the periphery of the vortex. These flux ropes propagate with the shear flow and eventually merge with the vortex. Over longer time scales, this basic scenario is repeated as the vortices drive new compressed current sheets. In the final stage, the vortices undergo a merging process that drives new compressed current sheets and flux ropes. Based on these simulations, a simple model is proposed that predicts the size of these flux ropes relative to their parent vortex. Both the relative sizes as well as the structure of the profiles across the vortex are in reasonable agreement with THEMIS observations at the Earth's low-latitude magnetopause.

Description: [1]  Using a traceable framework of idealised GCM experiments, a non-linear dependence of tropical precipitation pattern change on CO 2 forcing is identified. These non-linearities are relatively large and widespread throughout the tropics, and so should not be neglected in projections of future precipitation change. This has implications for the use of pattern-scaling and simple climate models to produce precipitation projections, and for physical understanding of precipitation change across forcing scenarios. The non-linearities can be understood by considering that processes which cause precipitation change, such as increasing moisture, a weakening circulation, and convergence zone shifts, interact in a non-linear manner even when individual processes arequasi-linear. Three driver interactions are identified: ‘warm-shift’, ‘warm-weak’ and ‘shift-weak’. Combined with Clausius-Clapeyron non-linearity in moisture increase, these interactions drive the non-linear pattern change. A strong convergence feedback response substantially amplifies the non-linearity. This analysis is limited to ocean regions, as mechanisms are simpler than over land.

Description: [1]  We present a new method to study harmonic waves in the low ionosphere (60 - 90 km) by detecting their effects on reflection of very low frequency (VLF) radio waves. Our procedure is based on amplitude analysis of reflected VLF radio waves recorded in real time, which yields an insight into the dynamics of the ionosphere at heights where VLF radio waves are being reflected. The method was applied to perturbations induced by the solar terminator motions at sunrises and sunsets. The obtained results show that typical perturbation frequencies found to exist in higher regions of the atmosphere are also present in the lower ionosphere, which indicates a global nature of the considered oscillations. In our model atmosphere, they turn out to be the acoustic and gravity waves with comparatively short and long periods, respectively.

Description: [1]  Development of the Antarctic Circumpolar Current (ACC) during the Cenozoic is controversial in terms of timing and its role in major climate transitions. Some propose that the development of the ACC was instrumental in the continental scale glaciation of Antarctica and climate cooling at the Eocene/Oligocene boundary. Here we present climate model results that show a coherent ACC was not possible during the Oligocene due to Australasian paleogeography, despite deep water connections through the Drake Passage and Tasman Gateway and the initiation of Antarctic glaciation. The simulations of ocean currents compare well to paleoenvironmental records relating to the physical oceanography of the Oligocene and provide a framework for understanding apparently contradictory dating of the initiation of the ACC. We conclude that the northward motion of the Australasian land masses and the reconfiguration of the Tasman Seaway and Drake Passage are necessary preconditions for the formation of a strong, coherent ACC.

Description: [1]  The M 7.7 Haida Gwaii earthquake radiated waves that likely dynamically triggered the M 7.5 Craig earthquake, setting two precedents. Firstly, the triggered earthquake is the largest dynamically triggered shear failure event documented to date. Secondly, the events highlight a connection between geologic structure, sedimentary troughs that act as waveguides, and triggering probability. The Haida Gwaii earthquake excited extraordinarily large waves within and beyond the Queen Charlotte Trough, which propagated well into mainland Alaska and likely triggering the Craig earthquake along the way. Previously, focusing and associated dynamic triggering have been attributed to unpredictable source effects. This case suggests that elevated dynamic triggering probabilities may exist along the many structures where sedimentary troughs overlie major faults, such as subduction zones’ accretionary prisms and transform faults’ axial valleys. Although data are sparse, I find no evidence of accelerating seismic activity in the vicinity of the Craig rupture between it and the Haida Gwaii earthquake.

Description: [1]  Abundant short-period, small-scale gravity waves have been identified in the mesosphere and lower thermosphere over Halley, Antarctica, via ground-based airglow image data. Although many are observed as freely-propagating at the heights of the airglow layers, new results under modeled conditions reveal that a significant fraction of these waves may be subject to reflections at altitudes above and below. The waves may at times be trapped within broad thermal ducts, spanning from the tropopause or stratopause to the base of the thermosphere (~140 km), which may facilitate long-range propagation (~1000 s of km) under favorable wind conditions.

Description: [1]  The capability of seasonal forecasting of global drought onset at local scales (1-degree) has been investigated using multiple climate models with 110 realizations. Climate models increase the global mean probability of drought onset detection from the climatology forecast by 31%-81%, but only increase equitable threat score by 21%-50% due to a high false alarm ratio. The multi-model ensemble increases the drought detectability over some tropical areas where individual models have better performance, but cannot help more over most extra-tropical regions. On average, less than 30% of the global drought onsets can be detected by climate models. The missed drought events are associated with low potential predictability and weak antecedent ENSO signal. Given the high false alarms, the reliability is very important for a skillful probabilistic drought onset forecast. This raises the question of whether seasonal forecasting of global drought onset is essentially a stochastic forecasting problem.

Description: [1]  New sub-auroral K -derived sector indices are proposed. They are based upon the K local geomagnetic activity indices from the planetary am network stations, and their derivation scheme draws directly from that of am indices. Four Magnetic Local Time (MLT) sectors are considered, leading to four different K -derived MLT-sector indices: the aσ Dawn (03-09 MLT), aσ Noon (09-15 MLT), aσ Dusk (15-21 MLT) and aσ Midnight (21-03 MLT) indices. They cover more than 4 solar cycles and, thus, allow robust statistical analysis. Statistical studies of the whole aσ data series and case studies for two geomagnetic storms are presented. These analyses clearly show that the four aσ have specific behaviors, and that it is possible to get insight into both the statistical properties of the physical processes responsible for the observed geomagnetic activity and contribution to the dynamics of a given storm.

Description: [1]  We survey the properties of electron pitch angle distributions in the magnetotail plasma sheet at a distance between 15 and 19 R E from the Earth, using data from the Cluster PEACE instrument. We limit our survey to those pitch angle distributions measured when the IMF had been steadily northward or steadily southward for the previous three hours. We find that, at sub-keV energies the plasma sheet electron pitch angle distribution has an anisotropy such that there is a higher differential energyflux of electrons in the (anti-) field-aligned directions. Fitting the measured pitch angle distributions with both a single and two component kappa distribution reveals that this anisotropy is the result of the presence of a second, cold, component of electrons that is observed more often than not, and occurs during both the northward and southward IMF intervals. We present evidence that suggests the cold electron component has an ionospheric, rather than magnetosheath, source and is linked tothe large scale field aligned current systems that couple the magnetosphere and ionosphere.

Description: [1]  Boreal-winter near-surface atmospheric circulations over the Hawaiian region are known to influence the state of the tropical Pacific and initiate the development of El Niño/Southern Oscillation (ENSO) events. Here we show that these same preceding near-surface circulations have an additional influence on the longitudinal position of the resultant ENSO-related sea-surface temperatures (SSTs) as well, with warm (cold) events systematically shifted to the east (west) of the typical SST anomalies. In influencing this positioning, these atmospheric circulations in turn modify the near and far-field climate responses to these SSTs such that during warm events, the typical ENSO-related responses east (west) of the dateline are generally enhanced (reduced); conversely, during cold events the typical ENSO-related responses are generally reduced (enhanced). The fact that the extratropical atmospheric circulations in question influence the asymmetry of ENSO extremes with a 12-month lead time carries important implications for predicting the socio-economic impacts of these events.

Description: [1]  Driver functions for the Earth's magnetosphere-ionosphere system are derived from physical principles. Two processes act simultaneously: a reconnection-coupled MHD generator G and a viscous interaction. G accounts for the dayside reconnection rate, the length of the reconnection X-line, and current-saturation limits for the solar-wind generator. Two viscous drivers are derived: Bohm viscosity B and the freestream-turbulence effect F. A problematic proxy effect is uncovered wherein the viscous driver functions also describe the strength of reconnection. Two magnetospheric-driver functions written in terms of upstream-solar-wind parameters are constructed: G+B and G+F. The driver functions are tested against 7 geomagnetic indices. The reaction of the geomagnetic indices to G+B and G+F is nonlinear: nonlinear versions of the driver functions are supplied. Applying the driver functions at multiple timesteps yields correlation coefficients of ~85% with the AE and Kp indices; it is argued that multiple timestepping removes high-frequency uncorrelated signal from the drivers. Autocorrelation-function analysis shows strong 1-d and 1-yr periodicities in the AE index that are not in the solar-wind driver functions; correspondingly, highpass and lowpass filtering finds uncorrelated signal at 1-d and 1-yr timescales. Residuals (unpredicted variance) between the geomagnetic indices and the driver functions are analyzed: the residuals are anti-correlated with the solar-wind velocity, the solar F 10.7 radio flux, and the solar-wind current-saturation parameter. Removing diurnal, semiannual, and annual trends from the indices improves their correlation with the solar-wind driver functions. Simplified versions of the driver functions are constructed: the simplified drivers perform approximately as well as the full drivers.

Description: [1]  Plasma properties of Saturn's pre-midnight tail region are surveyed using Cassini/CAPS ion observations from 2010. Only low-latitude (|lat| 〈 6°) intervals in which the CAPS viewing was roughly symmetric inward and outward around the corotation direction are used. Our numerical moments algorithm returns nonzero ion density for 70% (999) of the intervals selected. Of these, 642 had detectable water-group ion densities, and the remainder were dominantly, if not entirely, light ions. The derived plasma parameters are similar to those found in an earlier study for the post-midnight tail region, except that we find little evidence for the systematic outflows identified in that study, and we do find numerous significant inflow events. One such inflow is identified as a dipolarization event, the first reported plasma properties of such a structure at Saturn. A second, long-lasting event may be evidence for the existence at times of a quasi-steady reconnection region in the pre-midnight tail. The large majority of the plasma flows are found to be within 20° of the corotation direction, though with flow speeds significantly lower than full corotation. While the inflow events represent plausible evidence for internally-driven mass loss in the pre-midnight region, the absence of significant outflow events suggests that in the region surveyed here, tail reconnection has not yet proceeded to involve lobe field lines, so the disconnected plasma continues its general motion in the corotation direction.

Description: [1]  Following the first-time ionospheric imaging of a seismic fault, here we perform a case-study on retrieval of parameters of the extended seismic source ruptured during the great M9.0 Tohoku-oki earthquake. Using 1Hz ionospheric GPS data from the Japanese network of GPS receivers (GEONET) and several GPS satellites, we analyze spatio-temporal characteristics of co-seismic ionospheric perturbations and we obtain information on the dimensions and location of the sea surface uplift (seismic source). We further assess the criterion for the successful determination of seismic parameters from the ionosphere: the detection is possible when the line-of-sights from satellites to receivers cross the ionosphere above the seismic fault region. Besides, we demonstrate that the multi-segment structure of the seismic fault of the Tohoku-oki earthquake can be seen in high-rate ionospheric GPS-data. Overall, our results show that, under certain conditions, ionospheric GPS-derived TEC measurements could complement the currently working systems, or independent ionospherically-based system might be developed in the future.

Description: [1]  The non-dipolar portions of the Earth's main magnetic field constitute substantial differences between the two hemispheres. Beside the magnetic flux densities and patterns being different in the Northern Hemisphere (NH) and Southern Hemisphere (SH), also the offset between the invariant magnetic and the geographic poles is larger in the SH than in the NH. We investigated the effects of this magnetic field asymmetry on the high-latitude thermosphere and ionosphere using global numerical simulations, and compared our results with recent observations. While the effects on the high-latitude plasma convection are small, the consequences for the neutral wind circulation are substantial. The cross-polar neutral wind and ion drift velocities are generally larger in the NH than the SH, and the hemispheric difference shows a semidiurnal variation. The neutral wind vorticity is likewise larger in the NH than in the SH, with the difference probably becoming larger for higher solar activity. In contrast, the spatial variance of the neutral wind is considerably larger in the SH polar region, with the hemispheric difference showing a strong semidiurnal variation. Its phase is similar to the phase of the semidiurnal variation of the hemispheric magnitude differences. Hemispheric differences in ion drift and neutral wind magnitude are most likely caused partly by the larger magnetic flux densities in the near-polar regions of the SH, and partly by the larger offset between the invariant and geographic pole in the SH, while differences in spatial variance are probably just caused by the latter. We conclude that the asymmetry of the magnetic field, both in strength and in orientation, establishes substantial hemispheric differences in the neutral wind and plasma drift in the high-latitude upper atmosphere, which can help to explain observed hemispheric differences with EDI/Cluster and CHAMP.

Description: [1]  We have identified 8 events with double-belt structure in the outer radiation belt from 110 CME-driven magnetic storms and 223 CIR-driven storms during 1994 to 2003 based on the SAMPEX data sets. Among them, 3 cases are related to CME-driven magnetic storms and 5 cases are related CIR-driven storms. All double-belt structure events in the outer radiation belt are found during the recovery phase of a magnetic storm for both CME- and CIR-related events—they usually start to form within 3–4 days after the onset of the magnetic storm. The pre-conditions needed to form a double-belt structure, for all the CME-related events, are found to be high solar wind dynamic pressure ( P dy ) and southward IMF Bz; Nevertheless, for the CIR-related events, they are found to be associated with high speed stream with southward interplanetary magnetic field, which is enhanced by a suitably orientated B y component.It is further found that the flux distributions of the double-belt structure can be fitted well with a simply exponential decay function of L ∗ . Based on the RBC index, the proportion of the total number of 1.5-6.0 MeV electrons inside the position of maximum fluxes to that outside the maximum fluxes keeps rising during the double-belt period, which implies that the acceleration mainly occurs at regions inside the location of maximum fluxes. We suggest that the plasmapause and the strong wave-particle interactions with VLF and ULF waves near it, play an important role in the development of the double-belt structures.

Description: [1]  The solar minimum period during 2008–2009 was characterized by lower thermospheric density than the previous solar minimum, and lower than any previously measured. Recent work [ Solomon et al ., 2010; 2011] used the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model to show that the primary cause of density changes from 1996 to 2008 was a small reduction in solar extreme-ultraviolet irradiance (EUV), causing a decrease in thermospheric temperature and hence a contracted thermosphere. There are similar effects in the ionosphere, with most measurements showing an F -region ionosphere that is unusually low in density, and in peak altitude. This paper addresses the question of whether model simulations previously conducted, and their solar, geomagnetic, and anthropogenic inputs, produce ionospheric changes commensurate with observations. We conducted a 15-year model run and obtained good agreement with observations of the global mean thermospheric density at 400 km throughout the solar cycle, with a reduction of ~30% from the 1996 solar minimum to 2008–2009. We then compared ionosonde measurements of the mid-day peak density of the ionospheric F -region ( N m F 2 ) to the model simulations at various locations. Reasonable agreement was obtained between measurements and the model, supporting the validity of the neutral density comparisons. The global average N m F 2 was estimated to have declined between the two solar minima by ~15%. In these simulations, a 10% reduction of solar EUV plays the largest role in causing the ionospheric change, with a minor contribution from lower geomagnetic activity, and a very small additional effect from anthropogenic increase in CO 2 .

Description: [1]  The 1996 short-lived subaqueous eruption at the Karymsky caldera lake suddenly changed the composition of the lake water. The lake with the surface area of ~10 km 2 and a volume of ~0.5 km 3 became acidic, increased its salinity to ~1000 mg/kg, and became dominated by SO 4 2- and Ca 2+ . Since the eruption, the lake chemistry has evolved in a predictable manner described by simple box model. As a result of dilution by incoming SO 4 -Ca-Mg poor water, SO 4 , Ca and Mg concentrations follow a simple exponential decrease with a characteristic time close to the residence time of the lake. Na, K and Cl decrease relatively significantly slower indicating a continuing input of these constituents into the lake that was initiated during the eruption. Thus, the dynamics of two groups of lake water solutes can be predicted by a simple box model for water and solute mass balance.

Description: [1]  Our understanding of Earth's carbon-climate system depends critically upon interactions between rising atmospheric CO 2 , changing land use, and nitrogen limitation on vegetation growth. Using a global land model we show how these factors interact locally to generate the global land carbon sink over the past 200 years. Nitrogen constraints were alleviated by N 2 fixation in the tropics and by atmospheric nitrogen deposition in extra-tropical regions. Non-linear interactions between land-use change and land carbon and nitrogen cycling originated from three major mechanisms: (i) a sink foregone that would have occurred without land-use conversion; (ii) an accelerated response of secondary vegetation to CO 2 and nitrogen, and (iii) a compounded clearance loss from deforestation. Over time, these non-linear effects have become increasingly important and reduce the present-day net carbon sink by ~40% or 0.4 PgC yr -1 .

Description: [1]  The stability of the K-rich new aluminous (NAL) phase was examined on the join Na 1.00  Mg 2.00 Al 4.80 Si 1.15 O 12 –K 1.00  Mg 2.00 Al 4.80 Si 1.15 O 12 (Na100–K100) up to 144 GPa by X-ray diffraction in a laser-heated diamond-anvil cell. Single-phase K100 and Na50K50 NAL were formed up to the lower mantle conditions, and the NAL phase coexisted with the calcium ferrite-type phase at 120 GPa and 2300 K for the Na75K25 bulk composition. This is a striking contrast to the K-free (Na100) NAL that becomes unstable above 27 GPa at 1850 K, which suggests that potassium stabilizes NAL at significantly higher pressures. K-rich NAL may host potassium in the lower mantle that contains K 2 O more than 0.09 wt%. In addition, the NAL phase likely formed owing to partial melting in the ultralow velocity zone or because of a basal magma ocean. Future seismological observations may clarify whether NAL is a radiogenic heat source above the core–mantle boundary.

Description: [1]  We have developed a new wave scheme particularly aiming to provide better temperature fields with realistic variability for trajectory modeling of dehydration processes in the tropical tropopause layer (TTL). The new scheme includes amplitude-phase interpolation and amplification of waves in reanalysis data. Amplification factors are based on statistical variability differences between reanalysis data and radiosonde observations at 24 tropical locations during 1997-2013 boreal winters. We show that conventional linear interpolation of temperatures in the vertical coordinate degrades wave amplitudes and variability. Amplitude-phase interpolation in Fourier space greatly mitigates the problem found in linear interpolation. Furthermore, amplitudes of existing waves in reanalyses were amplified to generate realistic variability. In addition to improvements in variability, the scheme lowers cold point temperatures and raises cold point tropopause heights. Having realistic variability with the new approach will reduce uncertainties in simulations of TTL cirrus clouds and stratospheric water vapor.

Description: [1]  High ambient temperatures intensify photochemical production of tropospheric ozone, leading to concerns that global warming may exacerbate smog episodes. This widely-observed phenomenon has been termed the climate penalty factor (CPF). A variety of meteorological and photochemical processes have been suggested to explain why surface ozone increases on hot days. Here, we quantify an anthropogenic factor previously overlooked: the rise of ozone precursor emissions on hot summer days due to high electricity demand. Between 1997 and 2011, power plant emissions of NO x in the eastern U.S. increased by ~2.5-4.0%/°C, raising surface NO x concentrations by 0.10-0.25 ppb/°C. Given an ozone production efficiency (OPE) of ~8 mol/mol based on the 2011 NASA DISCOVER-AQ campaign, at least 1/3 of the CPF observed in the eastern U.S. can be attributed to the temperature dependence of NO x emissions. This finding suggests that controlling emissions associated with electricity generation on hot summer days can mitigate the CPF.

Description: [1]  We report the statistical features of sporadic sodium layers (SSLs) and the thermospheric enhanced sodium layers (TeSLs) observed by a lidar chain located at Beijing (40.2°N, 116.2°E), Hefei (31.8°N, 117.3°E), Wuhan (30.5°N, 114.4°E), and Haikou (19.5°N, 109.1°E). The average SSL occurrence rate was approximately 46.0, 12.3, 13.8, and 15.0 hr per SSL at Beijing, Hefei, Wuhan, and Haikou, respectively. However, the TeSLs occurred relatively infrequently and were more likely to appear at low and high latitudinal sites. Both the SSLs and TeSLs at four lidar sites showed evident summer enhancements and correlated well with Es ( f o E s  〉  4  MHz ). The co-observations of SSLs at three lidar site pairs, i.e., Hefei – Beijing, Hefei – Wuhan and Hefei – Beijing, indicated that a large-scale SSL extended horizontally for at least a few hundred kilometers and exhibited a tidal-induced modulation. Moreover, the SSLs were better correlated for the Hefei – Wuhan and Hefei – Haikou pairs than the Hefei – Beijing pair, which suggested a difference in the dynamical/chemical process in mesosphere and lower thermosphere (MLT) between the Beijing site and the other sites.

Description: [1]  A highly sensitive all-sky EMCCD airglow imager has been operative in Longyearbyen, Norway (78.1°N, 15.5°E) since October 2011. The imager obtains the 630.0 nm all-sky images with an exposure time of 4 sec, which is about 10 times shorter than the conventional cooled CCD airglow imagers. This new equipment allows us to image the on-going structuring of polar cap patches in 2D fashion. Here, we report a case in which faint undulations appeared along the trailing edge of patches propagating in the central polar cap. The separation between the fingers in the undulations was about 50–100 km and the e -folding time of their growth was ~5 min. We suggest that the gradient-drift instability (GDI) is one of the possible generation mechanisms of the undulating structures. The reasons for this interpretation are 1) the asymmetry in the preference of structuring between the leading and trailing edges is qualitatively consistent with the GDI mechanism, and 2) the linear growth rate of GDI calculated by using electron density estimates from simultaneous EISCAT Svalbard radar observations is roughly consistent with the observed growth time of the fingers. Such “unstable polar cap patches" could be important sources of seed irregularities, which would eventually be broken down to smaller-scale density perturbations affecting the trans-ionospheric satellite communications in the central polar cap.

Description: [1]  As the Polar spacecraft apogee precessed through the magnetic equator in 2001, Polar encountered numerous substorm events in the region between geosynchronous orbit and 10 Re geocentric distance; most of them in the plasma sheet boundary layers. Of these, a small number were recorded near the neutral sheet in the evening sector. Polar/TIDE provides a unique perspective on the lowest energy ion plasma, showing that these events exhibited a damped wavelike character, initiated by a burst of radially outward flow transverse to the local magnetic field at ~ 80 km/sec. They then exhibit strongly damped cycles of inward/outward flow with a period of several minutes. After one or two cycles, they culminated in a hot plasma electron and ion injection, quite similar to those observed at geosynchronous orbit. Cold plasmaspheric plasmas comprise the outward flow cycles, while the inward flow cycles contain counter-streaming field-parallel polar wind like flows. The observed wave-like structure, preceding the arrival of an Earthward moving substorm injection front, suggests an outward displacement driven by the inward motion at local times closer to midnight, that is, a “snowplow” effect. The damped in/out flows are consistent with interchange oscillations driven by the arrival at the observed local time by an injection originating at greater radius and local time.

Description: [1]  We derive a finite slip model for the 2013 M w 8.3 Sea of Okhotsk Earthquake (Z = 610 km) by inverting calibrated teleseismic P-waveforms. The inversion shows that the earthquake ruptured on a 10° dipping rectangular fault zone (140 km × 50 km) and evolved into a sequence of 4 large sub-events (E1-E4) with an average rupture speed of 4.0 km/s. The rupture process can be divided into two main stages. The first propagated south, rupturing sub-events E1, E2 and E4. The second stage (E3) originated near E2 with a delay of 12 s and ruptured northwards, filling the slip-gap between E1 and E2. This kinematic process produces an overall slip pattern similar to that observed in shallow swarms, except it occurs over a compressed time span of about 30s and without many aftershocks, suggesting that sub-event triggering for deep events is significantly more efficient than for shallow events.

Description: [1]  We report on seismic and petrological data that provide new constraints on the geological evolution of the Amerasia Basin. A seismic reflection profile across the Makarov Basin, located between the Mendeleev and Lomonosov ridges, shows a complete undisturbed sedimentary section of Mesozoic/Cenozoic age. In contrast to the Mendeleev Ridge, the margin of the Lomonosov Ridge is wide and shows horst and graben structures. We suggest that the Mendeleev Ridge is most likely volcanic in origin and support this finding with a 40 Ar/ 39 Ar isotopic age for a tholeitic basalt sampled from the central Alpha/Mendeleev Ridge. Seismic reflection data for the Makarov Basin show no evidence of compressional features consistent with the Lomonosov Ridge moving as a microplate in the Cenozoic. We propose that the Amerasia Basin moved as a single tectonic plate during the opening of the Eurasia Basin.

Description: [1]  Scaling relations for seismic moment M 0 , rupture area S , average slip D , and asperity size S a were obtained for large, great, and giant ( M w  = 6.7–9.2) subduction-zone earthquakes. We compiled the source parameters for seven giant ( M w  ~ 9) earthquakes globally for which the heterogeneous slip distributions were estimated from tsunami and geodetic data. We defined S a for subfaults exhibiting slip greater than 1.5 times D . Adding 25 slip models of 10 great earthquakes around Japan, we recalculated regression relations for 32 slip models: S  = 1.34 × 10 −10   M 0 2/3 , D  = 1.66 × 10 −7   M 0 1/3 , S a  = 2.81 × 10 −11   M 0 2/3 , and S a / S  = 0.2, where S and S a are in km 2 , M 0 is in Nm, and D is in m. These scaling relations are very similar to those obtained by Murotani et al. (2008) for large and great earthquakes. Thus, both scaling relations can be used for future tsunami hazard assessment associated with a giant earthquake.

Description: [1]  We observe the nucleation phase of in-plane ruptures in the laboratory. We show that the nucleation is composed of two distinct phases, a quasi-static and an acceleration stage, followed by dynamic propagation. We propose an empirical model which describes the rupture length evolution: the quasi-static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time. The transition from quasistatic to accelerating rupture is related to the critical nucleation length, which scales inversely with normal stress in accordance with theoretical predictions, and to a critical surfacic power, which may be an intrinsic property of the interface. Finally, we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions.

Description: [1]  Gravity anomalies derived from recent GRAIL data suggest the presence of early volume expansion of the Moon. The absence of identifiable thrust faults limits the total net contraction that has occurred. These observations provide constraints on the lunar thermal evolution which raise questions for giant impact origin of the Moon. To study the lunar expansion/contraction history, we perform 3D thermochemical mantle evolution models, with solidifying core overlain by a layer of ilmenite-bearing cumulates (IBC) resulting from mantle overturn after magma ocean solidification. Our models focus on the effects of the overturn-produced density stratification with a deep heat-producing element (HPE) distribution and a top insulating megaregolith layer. The deep HPE can cause an early expansion up to 1.5 km radius due to the heating of the deep mantle. This HPE distribution also reduces the present-day contraction by ~7 km. Compared to the models without overturn, an end-member model with a stable IBC-rich layer on the core-mantle boundary shows an overall present-day contraction as small as 1.1 km. The low thermal conductivity of megaregolith also affects the present-day contraction, reducing it by ~ 3 km.

Description: [1]  The interaction between the moons and the magnetosphere of giant planets sometimes gives rise to auroral signatures in the planetary ionosphere, called the satellite footprints. So far, footprints have been detected for Io, Europa, Ganymede and Enceladus. These footprints are usually seen as single spots. However, the Io footprint, the brightest one, displays a much more complex morphology made of at least three different spots and an extended tail. Here, we present Hubble Space Telescope FUV images showing evidence for a second spot in the Ganymede footprint. The spots separation distance changes as Ganymede moves latitudinally in the plasma sheet, as is seen for the Io footprint. This indicates that the processes identified at Io are universal. Moreover, for similar Ganymede System III longitudes, the distance may also vary significantly with time, indicating changes in the plasma sheet density. We identified a rapid evolution of this distance ~8 days after the detection of a volcanic outburst at Io, suggesting that such auroral observations could be used to estimate the plasma density variations at Ganymede.

Description: [1]  This paper presents a predictability study of the Madden-Julian Oscillation (MJO) that relies on combining empirical model reduction (EMR) with the “past-noise forecasting” (PNF) method. EMR is a data-driven methodology for constructing stochastic rotect low-dimensional models that account for nonlinearity, seasonality, and serial correlation in the estimated noise, while PNF constructs an ensemble of forecasts that accounts for interactions between (i) high-frequency variability (“noise”), estimated here by EMR; and (ii) the low-frequency mode (LFM) of MJO, as captured by singular-spectrum analysis (SSA). A key result is that — compared to an EMR ensemble driven by generic white noise — PNF is able to considerably improve prediction of MJO phase. When forecasts are initiated from weak MJO conditions, the useful skill is of up to 30 days. PNF also significantly improves MJO prediction skill for forecasts that start over the Indian Ocean.

Description: [1]  Accurate knowledge of the global distribution of magnetospheric chorus waves is essential for radiation belt modeling because it provides a direct link to understanding radiation belt losses and acceleration processes. In this paper, we report on newly developed models of the global distribution of chorus amplitudes based on in-situ measurements of IMF and solar wind parameters as well as geomagnetic indices using an artificial neural network technique. We find that solar wind speed and IMF B Z are the most influential parameters that affect the evolution of the magnetospheric chorus. The variations of chorus amplitudes in the outer ( L  ≥ 7) and in the inner (5 ≤  L  〈 7) regions, respectively, are well correlated with the variations of solar wind speed and IMF B Z . In addition, the solar wind parameter-based chorus model generally results in a slightly higher correlation between measured and modeled chorus amplitudes than any other models including geomagnetic indices AE, Kp, and Dst. The developed model shows that the chorus is amplified near the pre-noon sector during the geomagnetically disturbed conditions. With increasing southward IMF B Z the location of peak chorus amplitude moves from the pre-noon sector to the midnight sector, which is due to the enhanced electron injection near midnight. We also present a comparison of diffusive transport simulations for radiation belt electrons interacting with two newly developed chorus models, solar wind parameter-based and geomagnetic index-based chorus models. The comparison between two models shows that the modeling outside the plasmapause can affect the dynamic even inside the plasmasphere because the populations outside the plasmapause can act as seed population to radiation belt particles inside the plasmapause. One weakness of our chorus modeling is that it is trained during the early phase of solar cycle 24 where very few strong storms occurred. Therefore, our model might not be valid in reproducing the chorus activity under extremely disturbed conditions, which should be updated in the future once chorus measurements for such conditions become available.

Description: [1]  We investigate the storm-scale morphology of the magnetospheric magnetic field as well as underlying distributions of electric currents, equatorial plasma pressure and entropy for four Steady Magnetospheric Convection (SMC) events that occurred during the May 2000 and October 2011 magnetic storms. The analysis is made using the empirical geomagnetic field model TS07D, in which the structure of equatorial currents is not predefined and it is dictated by data. The model also combines the strengths of statistical and event-oriented approaches in mining data for the reconstruction of the magnetic field. The formation of a near-Earth minimum of the equatorial magnetic field in the midnight sector is inferred from data without ad hoc assumptions of a special current system postulated in earlier empirical reconstructions. In addition, a new SMC class is discovered where the minimum equatorial field is substantially larger and located closer to Earth. The magnetic field tailward of the minimumis also much larger, and the corresponding area of accumulated magnetic flux may occupy a very short tail region. The equatorial current and plasma pressure are found to be strongly enhanced far beyond geosynchronous orbit and in a broad local time interval covering the whole nightside region. This picture is consistent with independent recent statistical studies of the SMC pressure distributions, global MHD and kinetic RCM-E simulations. Distributions of the flux tube volume and entropy inferred from data reveal different mechanisms of the magnetotail convection crisis resolution for two classes of SMC events.

Description: [1]  Drifts in the Parker spiral interplanetary magnetic field are known to be an important component in the propagation of galactic cosmic rays, while they are thought to be negligible for Solar Energetic Particles (SEPs). As a result they have so far been ignored in SEP propagation modelling and data analysis. We examine drift velocities in the Parker spiral within single particle first-order adiabatic theory, in a local coordinate system with an axis parallel to the magnetic field. We show that, in the presence of scattering in interplanetary space, protons at the high end of the SEP energy range experience significant gradient and curvature drift. In the scatter-free case, drift due to magnetic field curvature is present. The magnitude of drift velocity increases by more than an order of magnitude at high heliographic latitudes compared to near the ecliptic; it has a strong dependence on radial distance r from the Sun, reaching a maximum at r ~1 AU at low heliolatitudes and r ~10 AU at high heliolatitudes. Due to the mass over charge dependence of drift velocities, the effect of drift for partially ionised SEP heavy ions is stronger than for protons. Drift is therefore likely to be a considerable source of cross field transport for high energy SEPs.

Description: [1]  We present a novel method for the automatic retrieval of local plasma density measurements from the Mars advanced radar for sub-surface and ionospheric sounding (MARSIS) active ionospheric sounder (AIS) instrument. The resulting large data set is then used to study the configuration of the Martian ionosphere at altitudes above ~300 km. An empirical calibration routine is used, which relates the local plasma density to the measured intensity of multiple harmonics of the local plasma-frequency oscillation, excited in the plasma surrounding the antenna in response to the transmission of ionospheric sounding pulses. Enhanced accuracy is achieved in higher-density ( n e  〉 150 cm − 3 ) plasmas, when MARSIS AIS is able to directly measure the fundamental frequency of the local plasma oscillation. To demonstrate the usefulness of this data set, the derived plasma densities are binned by altitude and solar zenith angle (SZA) in regions over weak ( nT) and strong ( nT) crustal magnetic fields, and we find clear and consistent evidence for a significant asymmetry between these two regions. We show that within the ~300-1200 km altitude range sampled, the median plasma density is substantially higher on the dayside in regions of relatively stronger crustal fields than under equivalent illuminations in regions of relatively weaker crustal fields. Conversely, on the nightside, median plasma densities are found to be higher in regions of relatively weaker crustal fields. We suggest that the observed asymmetry arises as a result of the modulation of the efficiency of plasma transport processes by the irregular crustal fields, and the generally horizontal draped interplanetary magnetic field (IMF).

Description: [1]  Global models of the Van Allen radiation belts usually include resonant wave-particle interactions as a diffusion process, but there is a large uncertainty over the diffusion rates. Here we present a new diffusion matrix for whistler mode chorus waves that can be used in such models. Data from seven satellites are used to construct 3,536 power spectra for upper and lower band chorus for 1.5 ≤  L ∗  ≤ 10, MLT, magnetic latitude 0 o  ≤ | λ m | ≤ 60 o and five levels of K p . Five density models are also constructed from the data. Gaussian functions are fitted to the spectra and capture typically 90% of the wave power. The frequency maxima of the power spectra vary with L ∗ and are typically lower than that used previously. Lower band chorus diffusion increases with geomagnetic activity and is largest between 21:00 and 09:00 MLT. Energy diffusion extends to a few MeV at large pitch angles 〉 60 o and at high energies exceeds pitch angle diffusion at the loss cone. Most electron diffusion occurs close to the geomagnetic equator (〈 12 o ). Pitch angle diffusion rates for lower band chorus increase with L ∗ and are significant at L ∗  = 8 even for low levels of geomagnetic activitywhile upper band chorus is restricted to mainly L ∗  〈 6. The combined drift and bounce averaged diffusion rates for upper and lower band chorus extend from a few keV near the loss cone up to several MeV at large pitch angles indicating loss at low energies and net acceleration at high energies.

Description: [1]  We report that radio science (RS) experiment onboard Mars Express (MEX) have observed three plasma layers in the nighttime ionosphere of Mars at altitudes ~80-100 km, ~120 km and ~160 km, which are reproduced by model calculation due to impact of meteoroid, solar wind proton and electron respectively. The densities of 21 ions (Mg + , Fe + , Si + , MgO + , MgCO 2 + , MgO 2 + , MgN 2 + , FeO + , FeO 2 + , FeN 2 + , FeCO 2 + , SiO + , SiCO 2 + , SiN 2 + , SiO 2 + , CO 2 + , N 2 + , O + , O 2 + , CO + , and NO + ) have been computed between altitude 50 km and 200 km. The model shows that all atmospheric ions (CO 2 + , N 2 + , O + , CO + , O 2 + and NO + ) are produced above 100 km due to solar wind electron and proton impact ionizations. The metallic ions are formed between 50 km and 100 km due to ablation of micrometeoroids. It is found that mass ~3.0 × 10 -4  g of incoming meteoroid is sufficient for meteor ablation and its characteristic flux ~4.0 × 10 -15  cm -2  s -1 could produce the nighttime metallic layer observed by MEX. The calculated electron densities are also compared with the occultation measurements made by Mars 4/5 in the nighttime ionosphere of Mars.

Description: [1]  Ultra Low Frequency (ULF) waves transfer energy in the Earth's magnetosphere through a variety of mechanisms that impact the Earth's ionosphere, radiation belts, and other plasma populations. Measurements of the electromagnetic portion of the energy transfer rate are an important source of information for assessing the importance of ULF waves relative to other energy transfer mechanisms as well as a diagnostic for studying the behavior of ULF waves. Using THEMIS satellite data, we examine the time averaged electromagnetic energy transfer rate, or Poynting vector, as a function of frequency and region of the magnetosphere; for this study, we focus on the direction and rate of energy transfer relative to the background magnetic field, comparing perpendicular and parallel transfer rates. This study extends earlier studies of the ULF wave Poynting vector that focused on narrower frequency ranges or specific regions of the magnetosphere; here, we consider the 3–50 mHz frequency range, all local time sectors, radial distances from 3 to 13 Re, and magnetic latitudes close to the equatorial plane. We measure time averaged Poynting vectors that range from  10 − 11 to 10 − 5 W / m 2 , with larger Poynting vector magnitudes occurring at largerradial distances and smaller frequencies. In every spatial region and frequency we examined, we found a large degree of scatter in both the Poynting vector magnitude and direction. The Poynting vector tends to be anisotropic at all frequencies,with more energy transferred along rather than across the background magnetic field. This preference for parallel energy transfer near the magnetic equator suggests that Joule dissipation in the ionosphere and the acceleration of auroral electrons are the largest sinks of ULF wave energy in the magnetosphere.

Description: [1]  Previous statistical studies have found a close relationship between high speed streams and high latitude geomagnetic activity. The speed by itself would increase the geoeffectivity of the solar wind. But it is also believed that pure Alfvénic fluctuations, often found in the trailing part of the streams, play a role in the solar wind driving of geomagnetic activity by amplifying the north-south component of the magnetic field ( B Z ), and thereby the dayside reconnection electric field. By automatically identifying slow and fast solar wind streams and by analysing them for more than one solar cycle, we aimed to study the relation between speed, Alfvénicity and B Z in the solar wind. It was found out that streams whose trailing parts are dominated by pure Alfvénic fluctuations, are the most geoeffective streams, on average. However, it is not the pure Alfvénic fluctuations themselves which cause the streams to be more geoeffective. There is only a variation of about 10 % in B Z due to the Alfvénicity of the fluctuations. Instead the streams are more geoeffective because the pure Alfvénic fluctuations tend to occur during high solar wind speed and strong IMF. There is a substantial variation within the solar cycle of how Alfvénic the solar wind streams are, and years with many extremely Alfvénic streams tends to have more days with moderately large geoeffectivity. The list of solar wind streams is included as extra material to this paper.

Description: [1]  Data Interpolating Empirical Orthogonal Functions (DINEOFs) are a data-based method for determining a few orthogonal basis functions that optimally reproduce a given data set. This technique is applied to meridional drift measurements performed by the Coupled Ion Neutral Dynamics Investigation (CINDI) onboard the Communication/Navigation Outage Forecasting System (C/NOFS) as well as electron density profiles derived from GPS Radio Occulations (RO) performed by COSMIC. The low densities of the equatorial ionosphere spanning 2009 - 2010 restricted quality drift measurements by CINDI to altitudes near perigee, limiting the local time coverage of measurements. Full local time descriptions may be obtained as perigee moves through all local times though this requires a minimum 67 day season. To increase the data coverage of the ionosphere CINDI data is supplemented with COSMIC GPS RO data. DINEOFs are applied to median meridional drift measurements as well as COSMIC measurements spanning 2009-10 and are used to make a best estimate of the equatorial ionosphere at locations not observed. The scattered distribution of COSMIC profiles as well as the physical relationship between meridional ion drifts and the distribution of density with altitude improve thequality of the reconstructions compared to using CINDI alone. The DINEOF reconstructions demonstrate that the annual anomomly of reduced ionospheric densities in June compared to December measured by COSMIC is coincident with a change in the meridional ion drifts at the geomagnetic equator measured by CINDI.

Description: [1]  It is well accepted that the propagation of electromagnetic ion cyclotron (EMIC) waves are bidirectional near their source regions and unidirectional when away from these regions. The generally believed source region for EMIC waves is around the magnetic equatorial plane. Here we describe a series of EMIC waves in the Pc1 (0.2-5 Hz) frequency band above the local He + cyclotron frequency observed in situ by all four Cluster spacecraft on 9 April 2005 at mid-magnetic latitudes (MLAT = ~33°-49°) with L = 10.7-11.5 on the dayside (MLT = 10.3-10.4). A Poynting vector spectrum shows that the wave packets consist of multiple groups of packets propagating bidirectionally, rather than unidirectionally, away from the equator, while the local plasma conditions indicate that the spacecraft are entering into a region sufficient for local wave excitation. One possible interpretation is that, while part of the observed waves are inside their source region, the others are either close enough to the source region, or mixed with the wave packets from multiple source regions at different latitudes.

Description: [1]  The Mojave Neovolcanic Province (MNVP), located in the Mojave block of southern California, comprises late Miocene to Quaternary small-volume basaltic centers. Geochemistry indicates an asthenospheric source for the MNVP beginning in the late Miocene, but no physical evidence of missing mantle lithosphere has been presented. We utilize receiver functions and ambient noise tomography to image the lithosphere beneath the Mojave block. Regionally, we find thin crust that thickens distal to sites of MNVP volcanism. Shear wave velocities between 40 and 75 km depth are consistent with the presence of mantle lithosphere in the southern Mojave block and very thin or missing mantle lithosphere to the north. With one exception, MNVP volcanoes lie along this sharp boundary. Our observations, together with the established geologic history and geochemistry of the MNVP, can be explained by small-scale edge-driven convection producing ongoing lithospheric removal within the Mojave block. Our results provide another example of lithospheric instability that occurs in response to rapid changes in mantle dynamics induced by major changes in tectonic plate geometry.

Description: [1]  Computational, Mössbauer and synchrotron radiation experiments arrive at disparate conclusions regarding the magnetic state of the high-pressure, hexagonal closed packed, phase of iron, which likely comprises the bulk composition of Earth's inner core. Using a non-magnetic, moissanite anvil cell together with a superconducting magnetometer, we measured the remanent magnetization of iron in response to applied magnetic fields under pressure up to 21.5 GPa at room temperature. Two independent experiments using different pressure transmission media reveal a higher remanent magnetization at 21.5 GPa than at initial conditions, which could be attributed to a distorted hexagonal closed packed phase grown during the martensitic transition. Upon both compression and decompression, the remanent magnetization of the body centered cubic phase increases several times over initial conditions while the coercivity of remanence remains mostly invariant with pressure.

Description: [1]  Numerous empirical studies have analyzed International Satellite Cloud Climatology Project (ISCCP) data and reached contradictory conclusions regarding the influence of solar-modulated galactic cosmic rays on cloud fraction and cloud properties. The Multiangle Imaging SpectroRadiometer (MISR) instrument on the Terra satellite has been in continuous operation for 13 years and thus provides an independent (and previously unutilized) cloud dataset to investigate purported solar–cloud links. Furthermore, unlike many previous solar–climate studies that report cloud fraction MISR measures albedo, which has clearer climatological relevance. Our long-term analysis of MISR data finds no statistically significant correlations between cosmic rays and global albedo or globally averaged cloud height, and no evidence for any regional or lagged correlations. Moreover, epoch superposition analysis of Forbush decreases reveals no detectable albedo response to cosmic ray decreases, thereby placing an upper limit on the possible influence of cosmic ray variations on global albedo of 0.0029 per 5% decrease. The implications for recent global warming are discussed.

Description: [1]  We studied decadal-scale climate control of zooplankton biogeography driven by Kuroshio Extension (KE) dynamics using long-term zooplankton data and an advection model driven with currents from the Earth Simulator eddy-resolving ocean model. Passive tracer model experiments indicated that warm-water species transported from the south were retained in the Kuroshio-Oyashio Extension (KOE) region during years with a weak KE. A 2.5-year lag in the North Pacific Gyre Oscillation (NPGO) index was significantly correlated with the KE strength and with warm-water species abundance. These findings indicate that climate signals from the central and eastern North Pacific propagated westward, influencing not only transport in the KOE region but also regional ecosystem variability. Because the NPGO controls important aspects of the transport dynamics and ecosystem variability in the eastern North Pacific, this study provides additional evidence that large-scale climate patterns drive coherent changes in ecosystems throughout the North Pacific by impacting regional-scale transport dynamics.

Description: [1]  Using a stable isotope tracer technique, we studied the exchange of methyl chloride (CH 3 Cl) and methyl bromide (CH 3 Br) between plants and the atmosphere in a tropical rainforest in Malaysia. Most plant species examined showed not only production but also consumption of CH 3 Cl with a large net emission overall. In contrast, CH 3 Br consumption was comparable to its production, so the net emission was small. The rates of CH 3 Cl and CH 3 Br consumption were highly correlated with each other, and their ratio was consistent with reported values in terrestrial ecosystems, where microorganisms are responsible for the consumption. Such microorganisms might participate in the consumption we observed, as the consumption rates were faster in saplings, whose leaves were generally covered by epiphytic microorganisms, than in healthy looking leaves of mature trees.

Description: [1]  Taiwan's active mountain belt is a spotlight for orogenic studies and was first used to test the critical-taper wedge mechanics. The concept of an orogenic wedge above a shallow detachment surface has been highly influential on current understanding of orogenic processes in Taiwan. However, the recent M L 6.2 and M L 6.5 2013 Nantou reverse-faulting earthquakes in central Taiwan nucleated below the proposed detachment indicating that active mountain building is occurring below the orogenic wedge. We estimate the coseismic slip distributions and fault geometry using the uniform stress drop slip inversions. The earthquakes occur on essentially the same 30° dipping fault plane ramping up from ~20 km depth near a cluster of 1999 Chi-Chi earthquake aftershocks to the shallow detachment and the Chi-Chi fault plane. The fault could be a deep extension of a mature shallow fault or a newly-developed deep ramp fault that is not reflected in the surface geology.

Description: [1]  Comparisons between climate models have found large differences in predictions for the albedo of forested regions with snow cover, leading to uncertainty in the strength of snow albedo feedbacks on climate change predicted by these models. To explore this uncertainty, three commonly used methods for calculating the albedo of vegetated surfaces are compared, taking observed snow and vegetation distributions as inputs. Surprisingly, all three methods produce similar results and compare reasonably well with observations over seasonally snow covered regions of the Northern Hemisphere. It appears that some climate models use unrealistic parameter values, and snow albedo masking need not be as large a source of uncertainty as it is in current climate projections.

Description: [1]  A classical paradigm for terrestrial climate variability involves remote sea-surface temperature forcing, communicated to receptor regions via atmospheric teleconnections. Here, the teleconnection link is abstracted in terms of Shannon's information-theoretic measure “channel capacity.” An upper bound on the channel capacity for DJF seasonal precipitation teleconnections with sea surface temperature in the NINO3.4 region, when both variables are tercile-quantized, is estimated as one bit, meaning that it is only marginally possible to distinguish reliably between two NINO3.4 input states on the basis of observed precipitation output amounts, the central tercile acting principally to degrade reliability. A relationship between the channel capacity in a continuous model and the correlation coefficient is established; the corresponding nonlinear transformation provides a useful shift in perspective on the communication of information as such via teleconnections.

Description: [1]  Although the persistently active Soufrière Hills Volcano (Montserrat, West Indies) is one of the most extensively studied active stratovolcanoes, a local Bouguer gravity map of the volcano and the island of Montserrat has yet to be constructed. We collected 157 new gravity data, which we analysed and inverted in order to constrain the island's subsurface density distribution. Our model results reveal high-density material beneath the centres of the extinct volcanic complexes – presumably related to exposed dome cores – while the volcanic flanks and the active Soufrière Hills Volcano are underlain by low-density material. Volcaniclastic deposits and subsurface melt aggregations, respectively, may explain these negative gravity anomalies. Our results are in good agreement with previous structural observations from seismic tomography, yet a higher spatial density of the gravity survey network has allowed us to additionally capture smaller, shallow-seated anomalies in the gravity field that relate to tectonic structures and fluvial filling deposits.