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We present a set of neural network models that reproduce the dynamics of electron fluxes in the range of 50 keV $\sim$ 1 MeV in the outer radiation belt. The Outer Radiation belt Electron Neural net model for Medium energy electrons(ORIENT-M) uses only solar wind conditions and geomagnetic indices as input. The models are trained on electron flux data from the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard Van Allen Probes, and they can reproduce the dynamic variations of electron fluxes in different energy channels. The model results show high coefficient of determination $R^2 \sim $ 0.78-0.92 on the test dataset, an out-of-sample 30-day period from February 25 to March 25 in 2017, when a geomagnetic storm took place, as well as an out-of-sample one year period after March 2018. In addition, the models are able to capture electron dynamics such as intensifications, decays, dropouts, and the Magnetic Local Time (MLT) dependence of the lower energy ($\sim <$ 100 keV )electron fluxes during storms. The models have reliable prediction capability and can be used for a wide range of space weather applications. The general framework of building our model is not limited to radiation belt fluxes and could be used to build machine learning models for a variety of other plasma parameters in the Earth's magnetosphere., Comment: under review

The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (~93deg inclination), nearly circular, low-Earth (~450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss cone bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (~90min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50keV to 5MeV electrons with deltaE/E<40% and a fluxgate magnetometer (FGM) on a ~72cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC to 5Hz Nyquist (nominally) with <0.3nT/sqrt(Hz) noise at 1Hz. The spinning satellites (T_spin~3s) are equipped with magnetorquers that permit spin-up/down and reorientation maneuvers. The spin axis is placed normal to the orbit plane, allowing full pitch-angle resolution twice per spin. An energetic particle detector for ions (EPDI) measures 250keV-5MeV ions, addressing secondary science. Funded initially by CalSpace and the University Nanosat Program, ELFIN was selected for flight with joint support from NSF and NASA between 2014 and 2018., Comment: Submitted to Space Science Reviews April 2020. 51 pages, 7 tables, 21 figures

The first-order Fermi acceleration of electrons requires an injection of electrons into a mildly relativistic energy range. However, the mechanism of injection has remained a puzzle both in theory and observation. We present direct evidence for a novel stochastic shock drift acceleration theory for the injection obtained with Magnetospheric Multiscale (MMS) observations at Earth's bow shock. The theoretical model can explain electron acceleration to mildly relativistic energies at high-speed astrophysical shocks, which may provide a solution to the long-standing issue of electron injection., Comment: 7 pages, 4 figures. Published in PRL

Magnetic reconnection is an energy conversion process important in many astrophysical contexts including the Earth's magnetosphere, where the process can be investigated in-situ. Here we present the first encounter of a reconnection site by NASA's Magnetospheric Multiscale (MMS) spacecraft in the magnetotail, where reconnection involves symmetric inflow conditions. The unprecedented electron-scale plasma measurements revealed (1) super-Alfvenic electron jets reaching 20,000 km/s, (2) electron meandering motion and acceleration by the electric field, producing multiple crescent-shaped structures, (3) spatial dimensions of the electron diffusion region implying a reconnection rate of 0.1-0.2. The well-structured multiple layers of electron populations indicate that, despite the presence of turbulence near the reconnection site, the key electron dynamics appears to be largely laminar., Comment: 4 pages, 3 figures, and supplementary material

We present multi-satellite observations indicating a strong correlation between large amplitude radiation belt whistler-mode waves and relativistic electron precipitation. On separate occasions during the Wind petal orbits and STEREO phasing orbits, Wind and STEREO recorded intense whistler-mode waves in the outer nightside equatorial radiation belt with peak-to-peak amplitudes exceeding 300 mV/m. During these intervals of intense wave activity, SAMPEX recorded relativistic electron microbursts in near magnetic conjunction with Wind and STEREO. The microburst precipitation exhibits a bursty temporal structure similar to that of the observed large amplitude wave packets, suggesting a connection between the two phenomena. Simulation studies corroborate this idea, showing that nonlinear wave--particle interactions may result in rapid energization and scattering on timescales comparable to those of the impulsive relativistic electron precipitation., Comment: This paper has been withdrawn by the authors

Based on over 4 years of Van Allen Probes measurements, an empirical model of radiation belt electron equatorial pitch angle distribution (PAD) is constructed. The model, developed by fitting electron PADs with Legendre polynomials, provides the statistical PADs as a function of L-shell (L = 1-6), magnetic local time, electron energy (~30 keV to 5.2 MeV), and geomagnetic activity (represented by the Dst index) and is also the first empirical PAD model in the inner belt and slot region. For megaelectron volt electrons, model results show more significant day-night PAD asymmetry of electrons with higher energies and during disturbed times, which is caused by geomagnetic field configuration and flux radial gradient changes. Steeper PADs with higher fluxes around 90° pitch angle and lower fluxes at lower pitch angles for higher energy electrons and during active times are also present, which could be due to electromagnetic ion cyclotron wave scattering. For hundreds of kiloelectron volt electrons, cap PADs are generally present in the slot region during quiet times and their energy-dependent features are consistent with hiss wave scattering, while during active times, cap PADs are less significant especially at outer part of slot region, which could be due to the complex energizing and transport processes. The 90°-minimum PADs are persistently present in the inner belt and appear in the slot region during active times, and minima at 90° pitch angle are more significant for electrons with higher energies, which could be a critical evidence in identifying the underlying physical processes responsible for the formation of 90°-minimum PADs.

Dayside interplanetary (IP) shock-induced injections are known to be a source of highly relativistic electrons in Earth's outer radiation belt, and are possibly the only source of greater than 1 megaelectronvolt electrons in the inner belt. The associated electron energization process is well understood and modeled. Recently, relativistic electron depletion echoes have also been associated with IP shocks, but the processes driving the depletions are less well understood. In this study, we investigate in detail two shock induced greater than 1 megaelectronvolt electron depletion events observed by the Van Allen Probes, March 17, 2015 and May 24, 2013, and draw similarities to night-side substorm related enhancements and depletions. Both events exhibit shock induced enhancements on one of the Van Allen Probes and depletions on the other in greater than 1 megaelectronvolt channels, such observations have not previously been reported. The depletion of greater than 1 megaelectronvolt electrons during the March 17, 2015 event is associated with enhancements in 10 second - 100 second kiloelectronvolt electrons on the same spacecraft. The depletion is consistent with the effects of a lack of seed electrons at larger radial distances combined with inward motion due to asymmetric compression by the shock impact. The immediate enhancements and depletions of 75 kiloelectronvolt - 2.6 greater than 1 megaelectronvolt electrons are explained by the local phase space density radial profile. Observations of electron flux dynamics during the May 24, 2013 event can also be explained by a lack of a seed population at larger radial distances, supported by butterfly distributions observed during the event. The electron's inward radial motion can be attributed to the inward propagating impulse also associated with the greater than 1 megaelectronvolt electron enhancements observed on the complementary probe, rather than global asymmetric compression. This causal mechanism has parallels to substorm related depletions. Alternatively, evidence is provided to attribute the sudden depletions to losses due to a sudden but brief inward motion of the magnetopause.

We present multipoint observations of earthward moving dipolarization fronts and energetic particle injections from NASAs Magnetospheric Multiscale mission with a focus on electron acceleration. From a case study during a substorm on 02 August 2015, we find that electrons are only accelerated over a finite energy range, from a lower energy threshold at approx. 7-9 keV up to an upper energy cutoff in the hundreds of keV range. At energies lower than the threshold energy, electron fluxes decrease, potentially due to precipitation by strong parallel electrostatic wavefields or initial sources in the lobes. Electrons at energies higher than the threshold are accelerated cumulatively by a series of impulsive magnetic dipolarization events. This case demonstrates how the upper energy cutoff increases, in this case from approx. 130 keV to >500 keV, with each depolarization/injection during sustained activity. We also present a simple model accounting for these energy limits that reveals that electron energization is dominated by betatron acceleration.

Two of the largest geomagnetic storms of the last decade were witnessed in 2015. On 17 March 2015, a coronal mass ejection-driven event occurred with a Dst (Disturbance Storm Time Ring Current Index) value reaching 223 nanoteslas. On 22 June 2015 another strong storm (Dst reaching 204 nanoteslas) was recorded. These two storms each produced almost total loss of radiation belt high-energy (E (Energy) greater than or approximately equal to 1 millielectronvolt) electron fluxes. Following the dropouts of radiation belt fluxes there were complex and rather remarkable recoveries of the electrons extending up to nearly 10 millielectronvolts in kinetic energy. The energized outer zone electrons showed a rich variety of pitch angle features including strong butterfly distributions with deep minima in flux at alpha equals 90 degrees. However, despite strong driving of outer zone earthward radial diffusion in these storms, the previously reported impenetrable barrier at L (L-shell magnetic field line value) approximately equal to 2.8 was pushed inward, but not significantly breached, and no E (Energy) greater than or approximately equal to 2.0 millielectronvolts electrons were seen to pass through the radiation belt slot region to reach the inner Van Allen zone. Overall, these intense storms show a wealth of novel features of acceleration, transport, and loss that are demonstrated in the present detailed analysis.

During the first months of observations, the Magnetospheric Multiscale Fly's Eye Energetic Particle Spectrometer instrument has observed several instances of electron acceleration up to greater than 100 keV while in the vicinity of the dayside reconnection region. While particle acceleration associated with magnetic reconnection has been seen to occur up to these energies in the tail region, it had not yet been reported at the magnetopause. This study reports on observations of electron acceleration up to hundreds of keV that were recorded on 19 September 2015 around 1000 UT, in the midst of an X-line crossing. In the region surrounding the X-line, whistler-mode and broadband electrostatic waves were observed simultaneously with the appearance of highly energetic electrons which exhibited significant energization in the perpendicular direction. The mechanisms by which particles may be accelerated via reconnection-related processes are intrinsic to understanding particle dynamics among a wide range of spatial scales and plasma environments.

An active storm period in June 2015 showed that particle injection events seen sequentially by the four (MagnetosphericMultiscale) MMS spacecraft subsequently fed the enhancement of the outer radiation belt observed by Van Allen Probes mission sensors. Several episodes of significant southward interplanetary magnetic field along with a period of high solar wind speed (Vsw 500kms) on 22 June occurred following strong interplanetary shock wave impacts on the magnetosphere. Key events on 22 June 2015 show that the magnetosphere progressed through a sequence of energy-loading and stress-developing states until the entire system suddenly reconfigured at 19:32 UT. Energetic electrons, plasma, and magnetic fields measured by the four MMS spacecraft revealed clear dipolarization front characteristics. It was seen that magnetospheric substorm activity provided a seed electron population as observed by MMS particle sensors as multiple injections and related enhancements in electron flux.

Energetic (greater than tens of keV) magnetospheric particle escape into the magnetosheath occurs commonly, irrespective of conditions that engender reconnection and boundary-normal magnetic fields. A signature observed by the Magnetospheric Multiscale (MMS) mission, simultaneous monohemispheric streaming of multiple species (electrons, H+, Hen+), is reported here as unexpectedly common in the dayside, dusk quadrant of the magnetosheath even though that region is thought to be drift-shadowed from energetic electrons. This signature is sometimes part of a pitch angle distribution evolving from symmetric in the magnetosphere, to asymmetric approaching the magnetopause, to monohemispheric streaming in the magnetosheath. While monohemispheric streaming in the magnetosheath may be possible without a boundary-normal magnetic field, the additional pitch angle depletion, particularly of electrons, on the magnetospheric side requires one. Observations of this signature in the dayside dusk sector imply that the static picture of magnetospheric drift-shadowing is inappropriate for energetic particle dynamics in the outer magnetosphere.

The Kelvin-Helmholtz (ICH) instability at the Earth's magnetopause is predominantly excited during northward interplanetary magnetic field (IMF). Magnetic reconnection due to KH waves has been suggested as one of the mechanisms to transfer solar wind plasma into the magnetosphere. We investigate KH waves observed at the magnetopause by the Magnetospheric Multlscale (MMS) mission; in particular, we study the trailing edges of KH waves with Alfvenic ion jets. We observe gradual mixing of magnetospheric and magnetosheath ions at the boundary layer. The magnetospheric electrons with energy up to 80 keV are observed on the magnetosheath side of the jets, which indicates that they escape into the magnetosheath through reconnected magnetic field lines. At the same time, the low-energy (below 100eV) magnetosheath electrons enter the magnetosphere and are heated in the field-aligned direction at the high-density edge of the jets. Our observations provide unambiguous kinetic evidence for ongoing reconnection due to KH waves.

We examined an electron flux dropout during the 12-14 November 2012 geomagnetic storm using observations from seven spacecraft: the two Van Allen Probes, Time History of Events and Macroscale Interactions during Substorms (THEMIS)-A (P5), Cluster 2, and Geostationary Operational Environmental Satellites (GOES) 13, 14, and 15. The electron fluxes for energies greater than 2.0 MeV observed by GOES 13, 14, and 15 at geosynchronous orbit and by the Van Allen Probes remained at or near instrumental background levels for more than 24 h from 12 to 14 November. For energies of 0.8 MeV, the GOES satellites observed two shorter intervals of reduced electron fluxes. The first interval of reduced 0.8 MeV electron fluxes on 12-13 November was associated with an interplanetary shock and a sudden impulse. Cluster, THEMIS, and GOES observed intense He+ electromagnetic ion cyclotron (EMIC) waves from just inside geosynchronous orbit out to the magnetopause across the dayside to the dusk flank. The second interval of reduced 0.8 MeV electron fluxes on 13-14 November was associated with a solar sector boundary crossing and development of a geomagnetic storm with Dst<100 nT. At the start of the recovery phase, both the 0.8 and 2.0 MeV electron fluxes finally returned to near prestorm values, possibly in response to strong ultralow frequency (ULF) waves observed by the Van Allen Probes near dawn. A combination of adiabatic effects, losses to the magnetopause, scattering by EMIC waves, and acceleration by ULF waves can explain the observed electron behavior.

We find evidence for hydrated material in the lunar regolith using "albedo protons" measured with the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO). Fluxes of these albedo protons, which are emitted from the regolith due to steady bombardment by high energy radiation (Galactic Cosmic Rays), are observed to peak near the poles, and are inconsistent with the latitude trends of heavy element enrichment (e.g., enhanced Fe abundance). The latitudinal distribution of albedo protons anti-correlates with that of epithermal or high energy neutrons. The high latitude enhancement may be due to the conversion of upward directed secondary neutrons from the lunar regolith into tertiary protons due to neutron-proton collisions in hydrated regolith that is more prevalent near the poles. The CRaTER instrument may thus provide important measurements of volatile distributions within regolith at the Moon and potentially, with similar sensors and observations, at other bodies within the Solar System.

During early November 2013, the magnetosphere experienced concurrent driving by a coronal mass ejection (CME) during an ongoing high-speed stream (HSS) event. The relativistic electron response to these two kinds of drivers, i.e., HSS and CME, is typically different, with the former often leading to a slower buildup of electrons at larger radial distances, while the latter energizing electrons rapidly with flux enhancements occurring closer to the Earth. We present a detailed analysis of the relativistic electron response including radial profiles of phase space density as observed by both Magnetic Electron and Ion Sensor (MagEIS) and Relativistic Electron Proton Telescope instruments on the Van Allen Probes mission. Data from the MagEIS instrument establish the behavior of lower energy (<1 MeV) electrons which span both intermediary and seed populations during electron energization. Measurements characterizing the plasma waves and magnetospheric electric and magnetic fields during this period are obtained by the Electric and Magnetic Field Instrument Suite and Integrated Science instrument on board Van Allen Probes, Search Coil Magnetometer and Flux Gate Magnetometer instruments on board Time History of Events and Macroscale Interactions during Substorms, and the low-altitude Polar-orbiting Operational Environmental Satellites. These observations suggest that during this time period, both radial transport and local in situ processes are involved in the energization of electrons. The energization attributable to radial diffusion is most clearly evident for the lower energy (<1 MeV) electrons, while the effects of in situ energization by interaction of chorus waves are prominent in the higher-energy electrons.