Shock-induced prompt relativistic electron acceleration in the inner magnetosphere

We present twin Van Allen Probes spacecraft observations of the effects of a solar wind shock impacting the magnetosphere on 8 October 2013. The event provides details both of the accelerating electric fields associated with the shock and the response of inner magnetosphere electron populations across a broad range of energies. During this period, the two Van Allen Probes observed shock effects from the vantage point of the dayside magnetosphere at radial positions of L=3 and L=5, at the location where shock-induced acceleration of relativistic electrons occurs. The extended (~1min) duration of the accelerating electric field across a broad extent of the dayside magnetosphere, coupled with energy-dependent relativistic electron gradient drift velocities, selects a preferred range of energies (3-4MeV) for the initial enhancement. Those electrons - whose drift velocity closely matches the azimuthal phase velocity of the shock-induced pulse - stayed in the accelerating wave as it propagated tailward and received the largest increase in energy. Drift resonance with subsequent strong ULF waves further accentuated this range of electron energies. Phase space density and positional considerations permit the identification of the source population of the energized electrons. Observations detail the promptness (<20min), energy range (1.5-4.5MeV), energy increase (~500keV), and spatial extent (L∗3.5-4.0) of the enhancement of the relativistic electrons. Prompt acceleration by impulsive shock-induced electric fields and subsequent ULF wave processes therefore comprises a significant mechanism for the acceleration of highly relativistic electrons deep inside the outer radiation belt as shown clearly by this event. Key Points Dual-spacecraft dayside observations quantify shock-induced effects Drift resonance with induced electric fields accelerates 3-4 MeV electrons Energy increase ~500 keV occurs in <20 min for 3 MeV at L∗3.8