Inner radiation belt particle acceleration and energy structuring by drift resonance with ULF waves during geomagnetic storms

Geomagnetic storms are frequently associated with the formation of multiple bands of energetic electrons inside the inner radiation belt at L = 1.1–1.9 and with prominent energy structures of protons inside the slot region at L = 2.2–3.5. These structures typically from 100 keV up to the MeV range result from coherent interactions of energetic particles with quasi‐monochromatic ultra‐low frequency (ULF) waves. These waves are induced by magnetospheric changes due to the arrival of dense solar material and related nightside injections of particles from the outer magnetosphere that destabilize field lines in the inner magnetosphere down to L = 1.1. Using low‐altitude data from the polar orbiting Demeter spacecraft, we perform case and statistical studies of these structures. We show that with such a spacecraft, these structures are best seen near the South Atlantic Anomaly because of lowering of the belt particle mirror point. As evidenced from ground measurements, energy bands are associated with quasi‐sinusoidal ULF Pc5 and Pc4 waves with periods in the 1000 s range for L = 1.1–1.9 and in the 60 s range for L = 2.2–3.5. Numerical simulations of the coherent drift resonance of energetic particles with ultra‐low frequency waves show how the particles are accelerated and how the observed structures build up.