Simulated Trapping of Solar Energetic Protons for the 8-10 March 2012 Geomagnetic Storm: Impact on Inner Zone Protons as Measured by Van Allen Probes

Solar energetic protons (SEPs) have been shown to contribute significantly to the inner zone trapped proton population for energies & lt;100 MeV and L & gt; 1.3 (Selesnick et al., 2007, https://doi. org/10.1029/2006sw000275). The Relativistic Electron Proton Telescope (REPT) on the Van Allen Probes launched 30 August 2012 observed a double-peaked (in L) inner zone population throughout the 7-year lifetime of the mission. It has been proposed that a strong SEP event accompanied by a CME-shock in early March 2012 provided the SEP source for the higher L trapped proton population, which then diffused radially inward to be observed by REPT at L & sim; 2. Here, we follow trajectories of SEP protons launched isotropically from a sphere at 7 Re in 15 s cadence fields from an Lyon-Fedder-Mobarry coupled to Rice Convection Model global magnetohydrodynamic (MHD) simulation driven by measured upstream solar wind parameters. The timescale of the interplanetary shock arrival is captured, launching a magnetosonic impulse propagating azimuthally along the dawn and dusk flanks inside the magnetosphere, shown previously to produce SEP trapping. The MHD-test particle simulation uses Geostationary Operational Environmental Satellite (GOES) proton energy spectra to weight the initial radial profile required for the radial diffusion calculation over the following 2 years. GOES proton measurements also provide a dynamic outer boundary condition for radial diffusion. A direct comparison with REPT measurements 20 months following the trapping event in March 2012 supports this novel combination of short-term and long-term evolution of the newly trapped protons.

Automated summary

Solar energetic protons (SEPs) significantly contribute to the inner zone trapped proton population. The Relativistic Electron Proton Telescope (REPT) observed a double-peaked inner zone population. A simulation of SEP protons launched from a sphere and a comparison with REPT measurements support the novel trapping process.