Mass Density Inferred From Toroidal Wave Frequencies and Energization of Low‐Energy Helium Ions During H‐Band EMIC Wave Interval

Abstract

Van Allen Probe A observed strongly enhanced hydrogen (H+) band electromagnetic ion cyclotron (EMIC) waves lasting >6 hr without helium (He+) band EMIC wave activity outside the plasmasphere on 23 February 2014. During the H‐band EMIC wave interval, multiharmonic toroidal waves were detected by the spacecraft. We estimated the local mass density using the observed toroidal wave frequencies and background electron number density. It was found that the average ion mass is close to 1. This indicates that the protons are the dominant ion species for the H‐band wave interval. From these observations, we suggest that the cold ion composition plays a major role in determining the spectral properties of EMIC waves. We also observed low‐energy He+ ion flux enhancements associated with the H‐band EMIC waves in the direction perpendicular to the background magnetic field. We discuss how the H‐band waves energize the low‐energy He+ ions in the perpendicular direction.

Energization of Cold Protons and Helium Ions by EMIC Waves in the Inner Magnetosphere: Hybrid Simulations

“…Our numerical analysis provides that the behavior of cold ions energized by EMIC waves occurs in two steps. In the first step, EMIC waves are excited in the strong H-band and weak He-band in the early stage of the simulation (Ω H t = 0-150) and energize cold H + and He + ions mostly in the direction perpendicular to the background magnetic field, which is consistent with recent observations in the inner magnetosphere (Kim et al, 2022). Our simulations show that the cold particle energization in this step is mainly due to the bulk motions of cold particles.…”

“…Kim et al. (2022) also observed that H‐band EMIC waves can energize cold protons and He + ions in the perpendicular direction and that the helium ions are more energized than the protons by the H‐band wave.…”

“…The authors suggested that cold He + ions (<1 eV) are transversely heated up to 10 eV through the interaction with EMIC waves. Kim et al (2022) also observed that H-band EMIC waves can energize cold protons and He + ions in the perpendicular direction and that the helium ions are more energized than the protons by the H-band wave. Lee et al (2021) statistically analyzed low-energy proton and helium ion flux enhancements associated with EMIC waves observed by Van Allen Probes in the inner magnetosphere.…”

“…The v ⊥ also depends on the wave frequency and the ratio of wave frequency to ion gyrofrequency (ω/Ω j ). Thus, both cold H + and He + ions can be energized in the perpendicular direction by only the H‐band EMIC wave without wave activity in the He‐band (Kim et al., 2022). We showed that the H‐band waves are dominant for the interval of Ω H t = 0–200 and that the perpendicular energies of both cold species are larger than the parallel energies.…”

Observation of Non‐Resonance Interactions Between Cold Protons and EMIC Waves of Different Polarizations in the Inner Magnetosphere

“…In addition, a close correlation between the occurrence of EMIC waves and cold He + ions heating was found in previous observations (K H Kim et al., 2021; Lee et al., 2021; D Young et al., 1981; J. C. Zhang et al., 2011). The cold He + ions can be energized by EMIC waves up to suprathermal energies (tens of eV to a few hundred eV) in the perpendicular direction to the background magnetic field (K. H. Kim et al., 2022; Omidi et al., 2010; Shi et al., 2020; Yuan et al., 2016). The energy of the cold He + ions is much lower than the typical minimum resonant energy of EMIC waves, thus, the energy transfer between cold He + ions and EMIC waves should also be a non‐resonant interaction mechanism, which has been explored in theory and numerical simulation (Berchem & Gendrin, 1985; Bortnik et al., 2010; Mauk, 1982; Omura et al., 1985).…”