Ultra-low-frequency waves for below threshold and thousand times threshold flux events

Abstract

A recent study by Shah et al. [Astrophys. Space Sci. 366(2), 22 (2021)] explored the effects of large-scale solar wind structures on relativistic electron fluxes in the slot region and categorized the flux events into four classes, including those with the events of maximum peak fluxes, TTF (thousand times threshold flux), and the events of smallest fluxes, BTF (below threshold flux). This study compares ultra-low-frequency (ULF) waves inside a magnetosphere and at the magnetopause boundary for the BTF and TTF events. It is found that during the TTF event, the electron radial drift velocity peaked very early at the position of an inner Van Allen radiation belt (L = 2–3). The delay between the radial velocity peak and the peak of the toroidal mode electric field was 12 min. For the BTF event, strong ULF waves were absent in the outer Van Allen radiation belt. In contrast, peak powers of ULF waves in the outer radiation belt approached 80 (mV/m)2/Hz for the TTF event, and this peak exceeded the peak power of the toroidal mode electric field at the magnetopause boundary (L = 11) for the BTF event. Our findings are critical for understanding the transport of ULF wave-driven charged particles from the magnetopause boundary to the inner magnetosphere.