Three radiation belt flux dropout events seen by the Relativistic Electron Proton Telescope soon after launch of the Van Allen Probes in 2012 (Baker et al., 2013a) have been simulated using the Lyon-Fedder-Mobarry MHD code coupled to the Rice Convection Model, driven by measured upstream solar wind parameters. MHD results show inward motion of the magnetopause for each event, along with enhanced ULF wave power affecting radial transport. Test particle simulations of electron response on 8 October, prior to the strong flux enhancement on 9 October, provide evidence for loss due to magnetopause shadowing, both in energy and pitch angle dependence. Severe plasmapause erosion occurred during~14 h of strongly southward interplanetary magnetic field B z beginning 8 October coincident with the inner boundary of outer zone depletion.

[1] Prior to 2003, there are two known cases where ultrarelativistic (^10 MeV) electrons appeared in the Earth's inner zone radiation belts in association with high speed interplanetary shocks: the 24 March 1991 and the less well studied 21 February 1994 storms. During the March 1991 event electrons were injected well into the inner zone on a timescale of minutes, producing a new stably trapped radiation belt population that persisted for $10 years. More recently, at the end of solar cycle 23, a number of violent geomagnetic disturbances resulted in large variations in ultrarelativistic electrons in the inner zone, indicating that these events are less rare than previously thought. Here we present results from a numerical study of shock-induced transport and energization of outer zone electrons in the 1-7 MeV range, resulting in a newly formed 10-20 MeV electron belt near L $ 3. Test particle trajectories are followed in time-dependent fields from an MHD magnetospheric model simulation of the 29 October 2003 storm sudden commencement (SSC) driven by solar wind parameters measured at ACE. The newly formed belt is predominantly equatorially mirroring. This result is in part due to an SSC electric field pulse that is strongly peaked in the equatorial plane, preferentially accelerating equatorially mirroring particles. The timescale for subsequent pitch angle diffusion of the new belt, calculated using quasi-linear bounce-averaged diffusion coefficients, is in agreement with the observed delay in the appearance of peak fluxes at SAMPEX in low Earth orbit. We also present techniques for modeling radiation belt dynamics using test particle trajectories in MHD fields. Simulations are performed using code developed by the Center for Integrated Space Weather Modeling.

A survey of 27 to 45 MeV proton measurements from the HEO‐3 satellite during the years 1998 through 2005 has been taken to describe variability in the outer part of the inner radiation belt and slot region (L = 2 to 3). Rapid (∼1‐day) changes are described as injection or loss events, characterized respectively by Gaussian or exponential L dependencies. The radial extent of both event types is correlated to the minimum Dst of associated magnetic storms, while the injection magnitude is correlated to the flux of associated interplanetary solar proton events. Changes in the maximal L of observed trapped protons are consistent with trapping limits estimated from magnetic field line curvature. The inward extent and energy independence of the observed loss events are inconsistent with field line curvature induced scattering in a static magnetic field. However, time‐dependent geomagnetic cutoff suppression, observed during magnetic storms, may be the cause of significant losses. Drift resonance with electric field impulses caused by rapid magnetospheric compression is the likely cause of both solar proton injections and radial shifts of preexisting trapped protons.

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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.