Hot Plasma Effects on the Cyclotron‐Resonant Pitch‐Angle Scattering Rates of Radiation Belt Electrons Due to EMIC Waves

Abstract

To investigate the hot plasma effects on the cyclotron‐resonant interactions between electromagnetic ion cyclotron (EMIC) waves and radiation belt electrons in a realistic magnetospheric environment, calculations of the wave‐induced bounce‐averaged pitch angle diffusion coefficients are performed using both the cold and hot plasma dispersion relations. The results demonstrate that the hot plasma effects have a pronounced influence on the electron pitch angle scattering rates due to all three EMIC emission bands (H+, He+, and O+) when the hot plasma dispersion relation deviates significantly from the cold plasma approximation. For a given wave spectrum, the modification of the dispersion relation by hot anisotropic protons can strongly increase the minimum resonant energy for electrons interacting with O+ band EMIC waves, while the minimum resonant energies for H+ and He+ bands are not greatly affected. For H+ band EMIC waves, inclusion of hot protons tends to weaken the pitch angle scattering efficiency of >5 MeV electrons. The most crucial differences introduced by the hot plasma effects occur for >3 MeV electron scattering rates by He+ band EMIC waves. Mainly due to the changes of resonant frequency and wave group velocity when the hot protons are included, the difference in scattering rates can be up to an order of magnitude, showing a strong dependence on both electron energy and equatorial pitch angle. Our study confirms the importance of including hot plasma effects in modeling the scattering of ultra‐relativistic radiation belt electrons by EMIC waves.

A Statistical Survey of Radiation Belt Dropouts Observed by Van Allen Probes

“…Precipitation is caused by wave‐particle interactions that induce pitch angle diffusion of electrons (e.g., Tu et al, ; Thorne, ; Ni et al, , ; Ni, Li, et al, ; Ni, Zou, et al, ). Among the various magnetospheric waves, electronmagnetic ion cyclotron (EMIC) waves are thought to play an important role in radiation belt dropouts due to its high resonant energy (mostly above 1 MeV) with electrons and its short loss timescale (can be as short as a few minutes; e.g., Cao et al, ; Meredith et al, ; Ni et al, ; Ni, Cao, et al, ; Summers et al, ). Magnetopause shadowing occurs either due to the solar wind compression of the magnetopause or the outward radial diffusion of electrons (e.g., Turner et al, ).…”

Precipitation of MeV and Sub‐MeV Electrons Due to Combined Effects of EMIC and ULF Waves

“…Using statistics of Van Allen Probes observations, Xiang et al () have recently found that the minimum energy of electron dropouts varies with equatorial pitch angle α 0 roughly like the theoretically expected E min ( α 0 ) for cyclotron resonance with hydrogen‐band EMIC waves (Cao et al, ; Kersten et al, ; Mourenas et al, ; Ni et al, ; Zhang et al, ) at L * ≤ 4 ( L * is the adiabatically invariant L ‐shell) but that it decreases below E min ( α 0 ) at L * =5. This indicates that low‐energy electron losses related to EMIC waves are mainly localized at high L * ≥ 5.…”

“…Two different physical mechanisms have been proposed to explain them: magnetopause shadowing loss enhanced by the outward radial diffusion after a strong solar wind pressure pulse (Shprits et al, ; Turner et al, ) and precipitation into the atmosphere through pitch‐angle scattering by electromagnetic ion cyclotron (EMIC) waves during disturbed periods (Blum et al, ; Bortnik et al, ; Hendry et al, ; Kersten et al, ; Mourenas et al, ; Sandanger et al, ; Summers & Thorne, ; Thorne & Kennel, ; Wang et al, ; Yu et al, ; Zhang et al, ). Since quasi‐linear diffusion by EMIC waves alone cannot produce dropouts for all MeV electrons, due to a lack of cyclotron resonance at high equatorial pitch angles (Cao et al, ; Kersten et al, ; Ni et al, ; Usanova et al, ), a combined scattering by EMIC and chorus (e.g., Agapitov et al, ) waves present on the same L ‐shell has been invoked (Mourenas et al, ; Zhang et al, ). But this kind of dropout can only occur above the minimum energy, E min , for cyclotron resonance with EMIC waves (Mourenas et al, ; Zhang et al, ).…”

“…Various studies have demonstrated that the minimum energy ( E min ) for cyclotron resonance with hydrogen‐band EMIC waves (left‐hand polarized and quasi‐parallel) is generally ≃1–3 MeV (Cao et al, ; Chen et al, ; Kersten et al, ; Mourenas et al, ; Ni et al, ; Summers & Thorne, ; Zhang et al, ), with low E min corresponding to typical proton concentrations η p >0.7 (Kersten et al, ). Helium‐band and oxygen‐band EMIC waves usually lead to higher E min , especially when taking into account hot plasma effects (Cao et al, ; Kersten et al, ; Ni et al, ; Summers & Thorne, ). However, electron precipitation associated with EMIC waves has often been observed down to smaller energies, ∼0.4–1 MeV (Blum et al, ; Hendry et al, ; Wang et al, ; Zhang et al, ).…”

“…An open question, however, is how EMIC waves can contribute to electron losses at sub‐MeV energies, that is, at significantly smaller energy than the minimum energy ( E min >1–2 MeV) of cyclotron resonance with typical EMIC waves (Cao et al, ; Kersten et al, ; Mourenas et al, ; Ni et al, ; Summers & Thorne, ; Zhang et al, ). Chen et al () have recently shown that intense (∼1 nT) helium‐band EMIC wave packets with sharp edges could produce such low‐energy electron losses through nonresonant scattering.…”