Effect of hot anisotropic He⁺ ions on the growth and damping of electromagnetic ion cyclotron waves in the inner magnetosphere

Abstract

Physics of electromagnetic ion cyclotron (EMIC) waves is complicated by inclusion of heavy ions. In particular, He+ ions in the magnetosphere have long been considered to play important roles. Motivated by recent observations, we examine the effect of the inclusion of hot anisotropic He+ ions in addition to the usual hot anisotropic protons. We solve the kinetic dispersion relation for this examination and find the following results. First, inclusion of hot anisotropic He+ ions leads to the growth of EMIC waves at frequencies below the He+ gyrofrequency (He band) and a reduction of the EMIC wave growth rates (or damping of the waves) at frequencies between the proton and He+ gyrofrequencies (H band). Second, this effect is more dramatic for higher temperatures of He+ that would play a role in damping EMIC waves for both frequency bands and especially for cases without a He+ temperature anisotropy. Lastly, the effect is more prominent for cold plasma dominant conditions such as the region inside the plasmasphere or plume than for hot proton dominant conditions such as the region outside the plasmasphere. We propose that this last effect can at least partially explain the satellite observations indicating the preferred (though not exclusive) occurrence of He band waves inside the plasmasphere for the times when hot anisotropic He+ ions are supplied from the plasma sheet and ring current.

Excitation of O+ Band EMIC Waves Through H+ Ring Velocity Distributions: Van Allen Probe Observations

“…EMIC waves in H + and He + bands are thought to be excited by ring current ions with temperature-anisotropy velocity distributions ( e.g., Cornwall, 1965;Kennel & Petschek, 1966;Kozyra et al, 1984;Lee et al, 2017;Xue et al, 1993;Yuan et al, 2016), which has been verified by satellite observations (e.g., Min et al, 2015).…”

“…EMIC waves in H + and He + bands are thought to be excited by ring current ions with temperature‐anisotropy velocity distributions ( e.g., Cornwall, ; Kennel & Petschek, ; Kozyra et al, ; Lee et al, ; Xue et al, ; Yuan et al, ), which has been verified by satellite observations (e.g., Min et al, ). Considering that EMIC waves in O + band have different properties in comparison with those in H + and He + bands, that is, O + band EMIC waves often have large wave normal angles and left‐hand or/and linear polarization (Saikin et al, ; Yu et al, ), Yu et al () have used linear growth theory to predict that O + band EMIC waves might be excited by protons with ring velocity distributions ( ∂f ⊥ / ∂v ⊥ > 0, where f ⊥ = f ( v ⊥ , v // = 0) is the perpendicular velocity distribution).…”

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

“…By assuming that the particle population can be described by bi‐Maxwellian distributions (Chen et al, ; Gary et al, ; Stix, ), the hot plasma dispersion relation of left‐hand parallel‐propagating EMIC waves in a multi‐ion plasma consisting of hot anisotropic protons and four cold particle species, that is, cold electrons, protons, He + ions, and O + ions, can be written as (e.g., Gary, ; Kozyra et al, ; Lee et al, ) …”

Effect of Hot He+ Ions on the Electron Pitch Angle Scattering Driven by H+, He+, and O+ Band EMIC Waves

“…However, the warm/hot (>1 keV) He + ions have been frequently observed during the EMIC emissions (Anderson & Fuselier, ; Young et al, ). In the presence of hot ions, the cold plasma approximation is invalid, and the kinetic effect of hot ions cannot be ignored (e.g., Chen et al, , ; Lee et al, ; Ni et al, ; Silin et al, ). Ni et al () considered the effect of hot protons on the electron pitch angle scattering.…”

“…Ni et al () considered the effect of hot protons on the electron pitch angle scattering. Lee et al () suggested that anisotropic hot He + ions probably contribute to the growth of He + band EMIC waves in the high‐density plasmasphere or plume. However, the effect of hot He + ions on the electron pitch angle scattering driven by EMIC waves remains unknown.…”