Strong enhancements of outer Van Allen belt electrons have been shown to have a clear dependence on solar wind speed and on the duration of southward interplanetary magnetic field. However, individual case study analyses also have demonstrated that many geomagnetic storms produce little in the way of outer belt enhancements and, in fact, may produce substantial losses of relativistic electrons. In this study, focused upon a key period in August–September 2014, we use GOES geostationary orbit electron flux data and Van Allen Probes particle and fields data to study the process of radiation belt electron acceleration. One particular interval, 13–22 September, initiated by a short‐lived geomagnetic storm and characterized by a long period of primarily northward interplanetary magnetic field (IMF), showed strong depletion of relativistic electrons (including an unprecedented observation of long‐lasting depletion at geostationary orbit) while an immediately preceding, and another immediately subsequent, storm showed strong radiation belt enhancement. We demonstrate with these data that two distinct electron populations resulting from magnetospheric substorm activity are crucial elements in the ultimate acceleration of highly relativistic electrons in the outer belt: the source population (tens of keV) that give rise to VLF wave growth and the seed population (hundreds of keV) that are, in turn, accelerated through VLF wave interactions to much higher energies. ULF waves may also play a role by either inhibiting or enhancing this process through radial diffusion effects. If any components of the inner magnetospheric accelerator happen to be absent, the relativistic radiation belt enhancement fails to materialize.

Early observations indicated that the Earth's Van Allen radiation belts could be separated into an inner zone dominated by high-energy protons and an outer zone dominated by high-energy electrons. Subsequent studies showed that electrons of moderate energy (less than about one megaelectronvolt) often populate both zones, with a deep 'slot' region largely devoid of particles between them. There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (more than five megaelectronvolts). Here we analyse an extended data set that reveals an exceedingly sharp inner boundary for the ultrarelativistic electrons. Additional, concurrently measured data reveal that this barrier to inward electron radial transport does not arise because of a physical boundary within the Earth's intrinsic magnetic field, and that inward radial diffusion is unlikely to be inhibited by scattering by electromagnetic transmitter wave fields. Rather, we suggest that exceptionally slow natural inward radial diffusion combined with weak, but persistent, wave-particle pitch angle scattering deep inside the Earth's plasmasphere can combine to create an almost impenetrable barrier through which the most energetic Van Allen belt electrons cannot migrate.

The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth's radiation belts. Observations (up to E~10 MeV) show clearly the behavior of the outer electron radiation belt at different timescales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi-MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L~4.0 ± 0.5). This reveals graphically that both "competing" mechanisms of multi-MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession.

Plasmaspheric hiss is an important wave mode for the dynamics of inner terrestrial magnetosphere plasma populations. It acts to scatter high‐energy electrons out of trapped orbits about Earth and into the atmosphere, defining the inner edge of the radiation belts over a range of energies. A low‐frequency component of hiss was recently identified and is important for its ability to interact with higher‐energy electrons compared to typically considered hiss frequencies. This study compares the statistical properties of low‐ and high‐frequency plasmaspheric hiss in the terrestrial magnetosphere, demonstrating that they are statistically distinct wave populations. Low‐frequency hiss shows different behavior in frequency space, different spatial localization (in magnetic local time and radial distance), and different amplitude distributions compared to high‐frequency hiss. The observed statistical properties of low‐frequency hiss are found to be consistent with recently developed theories for low‐frequency hiss generation. The results presented here suggest that careful consideration of low‐frequency hiss properties can be important for accurate inclusion of this wave population in predictive models of inner magnetosphere plasma dynamics.

The relativistic electrons in the inner radiation belt have received little attention in the past due to sparse measurements and unforgiving contamination from the inner belt protons. The high-quality measurements of the Magnetic Electron Ion Spectrometer instrument onboard Van Allen Probes provide a great opportunity to investigate the dynamics of relativistic electrons in the low L region. In this letter, we report the newly unveiled pitch angle distribution (PAD) of the energetic electrons with minima at 90 • near the magnetic equator in the inner belt and slot region. Such a PAD is persistently present throughout the inner belt and appears in the slot region during storms. One hypothesis for 90 • minimum PADs is that off 90 • electrons are preferentially heated by chorus waves just outside the plasmapause (which can be at very low L during storms) and/or fast magnetosonic waves which exist both inside and outside the plasmasphere.

No instruments in the inner radiation belt are immune from the unforgiving penetration of the highly energetic protons (tens of MeV to GeV). The inner belt proton flux level, however, is relatively stable; thus, for any given instrument, the proton contamination often leads to a certain background noise. Measurements from the Relativistic Electron and Proton Telescope integrated little experiment on board Colorado Student Space Weather Experiment CubeSat, in a low Earth orbit, clearly demonstrate that there exist sub-MeV electrons in the inner belt because their flux level is orders of magnitude higher than the background, while higher-energy electron (>1.6 MeV) measurements cannot be distinguished from the background. Detailed analysis of high-quality measurements from the Relativistic Electron and Proton Telescope on board Van Allen Probes, in a geo-transfer-like orbit, provides, for the first time, quantified upper limits on MeV electron fluxes in various energy ranges in the inner belt. These upper limits are rather different from flux levels in the AE8 and AE9 models, which were developed based on older data sources. For 1.7, 2.5, and 3.3 MeV electrons, the upper limits are about 1 order of magnitude lower than predicted model fluxes. The implication of this difference is profound in that unless there are extreme solar wind conditions, which have not happened yet since the launch of Van Allen Probes, significant enhancements of MeV electrons do not occur in the inner belt even though such enhancements are commonly seen in the outer belt.Key PointsQuantified upper limit of MeV electrons in the inner beltActual MeV electron intensity likely much lower than the upper limitMore detailed understanding of relativistic electrons in the magnetosphere

Measurements of inner radiation belt protons have been made by the Van Allen ProbesRelativistic Electron-Proton Telescopes as a function of kinetic energy (24 to 76 MeV), equatorial pitch angle, and magnetic L shell, during late 2013 and early 2014. A probabilistic data analysis method reduces background from contamination by higher-energy protons. Resulting proton intensities are compared to predictions of a theoretical radiation belt model. Then trapped protons originating both from cosmic ray albedo neutron decay (CRAND) and from trapping of solar protons are evident in the measured distributions. An observed double-peaked distribution in L is attributed, based on the model comparison, to a gap in the occurrence of solar proton events during the 2007 to 2011 solar minimum. Equatorial pitch angle distributions show that trapped solar protons are confined near the magnetic equator but that CRAND protons can reach low altitudes. Narrow pitch angle distributions near the outer edge of the inner belt are characteristic of proton trapping limits.

Using observations from NASA's Van Allen Probes, we study the role of sudden particle enhancements at low L shells (SPELLS) as a source of inner radiation belt electrons. SPELLS events are characterized by electron intensity enhancements of approximately an order of magnitude or more in less than 1 day at L < 3. During quiet and average geomagnetic conditions, the phase space density radial distributions for fixed first and second adiabatic invariants are peaked at 2 < L < 3 for electrons ranging in energy from ~50 keV to ~1 MeV, indicating that slow inward radial diffusion is not the dominant source of inner belt electrons under quiet/average conditions. During SPELLS events, the evolution of electron distributions reveals an enhancement of phase space density that can exceed 3 orders of magnitude in the slot region and continues into the inner radiation belt, which is evidence that these events are an important—and potentially dominant—source of inner belt electrons. Electron fluxes from September 2012 through February 2016 reveal that SPELLS occur frequently (~2.5/month at 200 keV), but the number of observed events decreases exponentially with increasing electron energy for ≥100 keV. After SPELLS events, the slot region reforms due to slow energy‐dependent decay over several day time scales, consistent with losses due to interactions with plasmaspheric hiss. Combined, these results indicate that the peaked phase space density distributions in the inner electron radiation belt result from an “on/off,” geomagnetic‐activity‐dependent source from higher radial distances.