Multiple loss processes of relativistic electrons outside the heart of outer radiation belt during a storm sudden commencement

Abstract

By examining the compression‐induced changes in the electron phase space density and pitch angle distribution observed by two satellites of Van Allen Probes (RBSP‐A/B), we find that the relativistic electrons (>2 MeV) outside the heart of outer radiation belt (L* ≥ 5) undergo multiple losses during a storm sudden commencement. The relativistic electron loss mainly occurs in the field‐aligned direction (pitch angle α < 30° or >150°), and the flux decay of the field‐aligned electrons is independent of the spatial location variations of the two satellites. However, the relativistic electrons in the pitch angle range of 30°–150° increase (decrease) with the decreasing (increasing) geocentric distance (|ΔL| < 0.25) of the RBSP‐B (RBSP‐A) location, and the electron fluxes in the quasi‐perpendicular direction display energy‐dispersive oscillations in the Pc5 period range (2–10 min). The relativistic electron loss is confirmed by the decrease of electron phase space density at high‐L shell after the magnetospheric compressions, and their loss is associated with the intense plasmaspheric hiss, electromagnetic ion cyclotron (EMIC) waves, relativistic electron precipitation (observed by POES/NOAA satellites at 850 km), and magnetic field fluctuations in the Pc5 band. The intense EMIC waves and whistler mode hiss jointly cause the rapidly pitch angle scattering loss of the relativistic electrons within 10 h. Moreover, the Pc5 ULF waves also lead to the slowly outward radial diffusion of the relativistic electrons in the high‐L region with a negative electron phase space density gradient.

Electron flux dropouts at Geostationary Earth Orbit: Occurrences, magnitudes, and main driving factors

“…In which energy ranges? Losses due to field line stretching near midnight during strong disturbances [ Artemyev et al , ], as well as precipitation due to intense EMIC waves, occur preferentially at high (MeV) energies [ Summers and Thorne , ; Blum et al , ; Yu et al , ] and could contribute to the higher magnitudes and occurrences of dropouts at such high energies as compared with the range E ∼ 0.1–0.5 MeV. However, strong dropouts (i.e., extending up to equatorial pitch angles ∼90°) at relatively moderate energies ∼1.3–2.65 MeV induced by EMIC waves would require intense and widespread waves at frequencies very close to the helium ion gyrofrequency [ Ukhorskiy et al , ], which does not seem to be a very usual situation [ Kersten et al , ] due to strong damping [ Chen et al , ].…”

“…It indicates that some of the MeV electron dropouts are likely partially governed by some mechanisms different from the mechanisms prevalent at lower energy. The much steeper electron flux decrease observed during some MeV dropouts may be produced by field line curvature effects near midnight during strong magnetic disturbances [ Artemyev et al , ], or they may correspond to strong precipitation induced by combined EMIC and chorus waves [ Summers and Thorne , ; Blum et al , ; Gao et al , ; Yu et al , ; Mourenas et al , ]—two loss processes which are both strongly energy dependent.…”

“…Taken as a whole, our statistical results suggest that radial diffusion coupled with magnetopause shadowing should not be the sole (nor maybe the main) driving factor for all these dropouts. At relativistic energies above 1 MeV, precipitation induced by EMIC and chorus (or hiss) waves is probably also effective [ Blum et al , ; Gao et al , ; Yu et al , ; Mourenas et al , ], as well as scattering due to increased field line curvature during disturbances [ Artemyev et al , ]. At intermediate energies 0.1 < E < 1 MeV, the observed dropouts are probably mostly due to radial diffusion coupled with magnetopause shadowing, related to sudden increases of solar wind dynamic pressure or density, although chorus scattering should also play some role.…”

“…The above results suggest that radial diffusion coupled with magnetopause shadowing may not be the sole, nor maybe the main, driving factor for dropouts. At energies above 1 MeV, strong precipitation induced by EMIC and chorus (or hiss) waves may also be effective [ Blum et al , ; Gao et al , ; Yu et al , ; Mourenas et al , ], as well as scattering due to increased field line curvature [ Artemyev et al , ]—two processes that should usually take place during periods of increased southward IMF or S Y M ‐ H and that act preferentially at energies E > 1–2 MeV. At low energy E ≤300 keV, the observed dropouts can be partially due to chorus wave scattering [ Lam et al , ; Mourenas et al , ].…”

Effects of ULF waves on local and global energetic particles: Particle energy and species dependences

“…It contains a large number of energetic electrons and protons in the energy range from tens of keV to several MeV. The fluxes of the outer radiation belt particles are frequently changed during intense interplanetary (IP) disturbances and/or geomagnetic activities (storms and substorms) [ Li et al ., ; Baker et al ., ; Reeves et al ., ; Li et al ., , , ; Li et al ., ; Yu et al ., ; Tu et al ., ; Su et al ., ; Xiao et al ., ; Yang et al ., ; Yu et al ., , ]. Some changes in the energetic particle fluxes are associated with ULF waves [ Kokubun et al ., ; Kremser et al ., ; Takahashi et al ., ; Sarris et al ., ].…”