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We compute quasilinear diffusion rates due to pitch angle scattering by various mechanisms in the Earth's electron radiation belts. The calculated theoretical lifetimes are compared with observed decay rates, and we find excellent qualitative agreement between the two. The overall structure of the observed lifetime profiles as a function of energy and L is largely due to plasmaspheric hiss and Coulomb scattering. The results also reveal a local minimum in lifetimes in the inner zone at lower energy ( ∼50 keV), attributed to enhanced scattering via ground‐based very low frequency transmitters, and a reduction in lifetimes at higher L and energy ( >1 MeV), attributed to enhanced electromagnetic ion cyclotron wave scattering. In addition, we find significant quantitative disagreement at L, Key Points Electron scattering by Coulomb collisions, hiss, EMIC, and VLF transmitter waves is demonstrated in observed loss timescalesThe energy‐ L structure of the observed electron lifetimes is in excellent qualitative agreement with quasilinear theoryTheoretical lifetimes in the inner region ( L

Using recent data from NASA's Van Allen Probes, we estimate the quiet time radial diffusion coefficients for electrons in the inner radiation belt (L

We use Van Allen Probes data to investigate the responses of tens of keV to 2 MeV electrons throughout a broad range of the radiation belts (2.5 ≤ L ≤ 6.0) during 52 geomagnetic storms from the most recent solar maximum. Electron storm time responses are highly dependent on both electron energy and L shell. Tens of keV electrons typically have peak fluxes in the inner belt or near-Earth plasma sheet and fill the inner magnetosphere during storm main phases. Approximately 100 to ~600 keV electrons are enhanced in up to 87% of cases around L~3.7, and their peak flux location moves to lower L shells during storm recovery phases. Relativistic electrons (≥~1 MeV) are nearly equally likely to produce enhancement, depletion, and no-change events in the outer belt. We also show that the L shell of peak flux correlates to storm magnitude only for hundreds of keV electrons.

From a survey of the first nightside season of NASA's Van Allen Probes mission (December 2012 to September 2013), 47 energetic (tens to hundreds of keV) electron injection events were found at L shells ≤ 4, all of which are deeper than any previously reported substorm-related injections. Preliminary details from these events are presented, including how all occurred shortly after dipolarization signatures and injections were observed at higher L shells, how the deepest observed injection was at L ~ 2.5, and, surprisingly, how L ≤ 4 injections are limited in energy to ≤250 keV. We present a detailed case study of one example event revealing that the injection of electrons down to L ~ 3.5 was different from injections observed at higher L and likely resulted from electrons interacting with a fast magnetosonic wave in the Pi2 frequency range inside the plasmasphere. These observations demonstrate that injections occur at very low L shells and may play an important role for inner zone electrons.

We present Van Allen Probe observations of electrons in the inner radiation zone. The measurements were made by the Energetic Particle, Composition, and Thermal Plasma/Magnetic Electron Ion Spectrometer (MagEIS) sensors that were designed to measure electrons with the ability to remove unwanted signals from penetrating protons, providing clean measurements. No electrons >900 keV were observed with equatorial fluxes above background (i.e., >0.1 el/(cm2 s sr keV)) in the inner zone. The observed fluxes are compared to the AE9 model and CRRES observations. Electron fluxes

We report large directional anisotropies of >60 MeV protons using instrumentation on the Van Allen Probes. The combination of a spinning satellite and measurements from the Relativistic Proton Spectrometer instruments that are insensitive to protons outside the instrument field of view together yield a new look at proton radial gradients. The relatively large proton gyroradius at 60 MeV couples with the radial gradients to produce large (maximum ~10:1) flux anisotropies depending on (i) whether the proton guiding center was above or below the Van Allen Probes spacecraft and (ii) the sign of the local flux gradient. In addition to these newly measured anisotropies, below ~2000 km we report a new effect of systematically changing minimum altitude on some proton drift shells that further modulates the anisotropy caused by the atmosphere. This discovery may offer a new way of monitoring changes to the loss of inner belt protons into the Earth's atmosphere.

Using data from NASA's Van Allen Probes, we have identified a synchronized exponential decay of electron flux in the outer zone, near L* = 5.0. Exponential decays strongly indicate the presence of a pure eigenmode of a diffusion operator acting in the synchronized dimension(s). The decay has a time scale of about 4 days with no dependence on pitch angle. While flux at nearby energies and L* is also decaying exponentially, the decay time varies in those dimensions. This suggests the primary decay mechanism is elastic pitch angle scattering, which itself depends on energy and L*. We invert the shape of the observed eigenmode to obtain an approximate shape of the pitch angle diffusion coefficient and show excellent agreement with diffusion by plasmaspheric hiss. Our results suggest that empirically derived eigenmodes provide a powerful diagnostic of the dynamic processes behind exponential decays.

Relativistic electron observations near geostationary orbit routinely show pitch angle distributions peaked away from 90 degrees. These “butterfly” distributions are consistent with magnetic drift shell splitting combined with a radial flux gradient. During magnetic storms, nature adds pitch angle scattering to split drift shells, breaking all three adiabatic invariants of the particle's motion. Therefore, some degree of anomalous radial diffusion is likely, and cross terms between the gyration and drift invariants and between the bounce and drift invariants arise. Using typical assumptions about the pitch angle scattering and the magnetic field topology, we calculate these anomalous diffusion coefficients near geostationary orbit. We show that the anomalous radial diffusion can exceed that due to more traditional drift-resonant wave-particle interactions. We also show that the neglected cross terms, particularly the bounce-drift cross term, can be significant. These results suggest necessary additions to some global electron radiation belt simulations.

The interplanetary electromagnetic fields are generally considered the drivers of the storm-time terrestrial ring current. Recently, the solar wind density has been advocated as an additional driver. Solar wind density partially determines the density in the plasma sheet, which, in turn, is the likely source for the ring current. Therefore, the solar wind density may drive the ring current by enhancing the ring current source plasma. Some studies, using a few years of data, have found a strong statistical signal for a solar wind density driver with a few hours lag prior to the maximum ring current intensity, as measured by a minimum in Dst. However, we show, using a much larger database of storms, that no consistent role for density alone is evident on time scales of a few hours. The previously reported statistical signal seems to be isolated to a particular interval from November 1994 through September 1995.

[1] During January, 2005, there were several large X-class solar flares and associated solar energetic particle (SEP) events. Coincidentally, the MINIS balloon campaign had multiple payloads aloft in the stratosphere above Antarctica measuring dc electric fields, conductivity and x-ray flux. One-to-one increases in the electrical conductivity and decreases to near zero of both the vertical and horizontal electric field components were observed in conjunction with an increase in particle flux at SEP onset. Combined with an atmospheric electric field mapping model, these data are consistent with a shorting out of the global electric circuit and point toward substantial ionospheric convection modifications. Additionally, two subsequent, rapid changes were detected in the vertical electric field component several hours after SEP onset. These changes result in similar fluctuations in the calculated vertical current density. We will describe how rigidity cut-off dynamics may be crucial in understanding these sudden jumps in the vertical electric field.

[1] A remarkable correlation between the inner edge of the outer radiation belt electrons and the innermost plasmapause location is demonstrated by using 12 years of measurements of MeV electrons from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and the Combined Radiation and Release Experiment Satellite (CRRES) during its entire mission (July 1990–October 1991). An empirical model of the plasmapause location, Lpp, by O'Brien and Moldwin (2003) as well as the measured Lpp during the CRRES mission is used for the comparison. This correlation suggests that the innermost Lpp is the innermost limit of the outer radiation belt penetration. We discuss the possible scenarios that might explain this correlation.

[1] Bursty precipitation of relativistic electrons has been implicated as a major loss process during magnetic storms. One type of precipitation, microbursts, appears to contain enough electrons to empty the prestorm outer radiation belt in approximately a day. During storms that result in high fluxes of trapped relativistic electrons, microbursts continue for several days into the recovery phase, when trapped fluxes are dramatically increasing. The present study shows that this apparent inconsistency is resolved by observations that the number of electrons lost through microbursts is 10–100 times larger during the main phase than during the recovery phase of several magnetic storms chosen by the Geospace Environment Modeling (GEM) program.

[1] We use a database of CRRES in situ observations of plasmapause crossings to build empirical models of the plasmapause location as a function of geomagnetic indices. Previous models used the maximum value in Kp during the hours to days leading up to the plasmapause crossing. We find that a recent maximum in AE or minimum in Dst provides a better model of the plasmapause radius than does maximum Kp. AE and Dst measure specific current systems (the auroral electrojet and ring current, respectively). The AE model suggests that substorms may be involved in the erosion of the plasmapause. The Dst model suggests that ring current may be formed by the same electric field that erodes the plasmapause. In more complex models, Kp and AE can describe local time structure in the plasmapause, with the furthest plasmapause before dawn at quiet times, premidnight at active times.

[1] We test the hypothesis that abrupt cessation of solar wind driving results in a different recovery profile for Dst than does a slow cessation. The difference arises when abrupt cessation of convection causes a large portion of the asymmetric ring current to be trapped, whereas continued, weakened convection permits a rapid initial loss of ring current particles through the dayside magnetopause. Specifically, we compare the initial recovery of the hourly Dst index for storms with gradual versus abrupt northward turning of the interplanetary magnetic field (IMF). We consider only storms with minimum Dst between 150 nT and 300 nT. We show that storms with abrupt northwardturningsshowthesamerecoveryinthefirst6hours or slightly more recovery than do the storms with gradual northward turnings. Our results contradict the hypothesis that the rate of northward turning (i.e., shutoff of convection) largely determines the initial rate of recovery in Dst. INDEX TERMS: 2778 Magnetospheric Physics: Ring current; 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions; 2760 Magnetospheric Physics: Plasma convection; 2730 Magnetospheric Physics: Magnetosphere—inner; 2708 Magnetospheric Physics: Current systems (2409). Citation: O’Brien, T. P., R. L. McPherron, M. W. Liemohn, Continued convection and the initial recovery of Dst, Geophys. Res. Lett., 29(23), 2143, doi:10.1029/2002GL015556, 2002.