Wave‐particle interactions near Ω_He+ observed on board GEOS 1 and 2: 2. Generation of ion cyclotron waves and heating of He⁺ ions

Abstract

This work is a continuation of paper 1 (Young et al., 1981) and is devoted to the generation process of ion cyclotron waves (ICWs) and the acceleration of He+ ions up to suprathermal energies. Simultaneous measurements are used from the ion composition experiment (0 < E < 16 keV), the energetic particle experiment (24 < E < 3 300 keV), and the ULF wave experiment (0.2–10 Hz) on board the GEOS 1 and GEOS 2 spacecraft. General characteristics of the local time distribution of ICWs will be presented and compared with those of the thermal anisotropy of energetic protons and the He+ abundance. Further calculations of the convective growth rate are conducted by applying two different methods, both of which are based upon the measured proton fluxes. The generation conditions of the ICWs in the presence of He+ ions will be investigated and three possible explanations will be discussed: (1) enhanced convection growth rates, (2) lowering of the threshold for absolute instabilities, and (3) change of the ICWs ray path (laser‐like effect). Finally, it is shown that the flux of suprathermal He+ ions is modulated at the ICW frequency. Owing to nonlinear effects, part of the energy of the energetic protons is transfered via the ICWs to the He+ ions that are essentially accelerated in the direction perpendicular to the static magnetic field. Then in the otherwise collisionless plasma the friction between energetic anisotropic protons and thermal He+ ions is achieved through the ICWs.

Excitation of EMIC waves detected by the Van Allen Probes on 28 April 2013

“…The two-point, frequent measurements of the Van Allen Probes allow us to pinpoint local field and plasma conditions for EMIC waves in the inner magnetosphere. New insight into the excitation of EMIC waves from this report is as follows: [Kennel and Petschek, 1966;Roux et al, 1982]. The local wave occurrences are reconfirmed with the testing of linear theory, even though it has intrinsic limitations (see below).…”

“…Although EMIC waves have been intensively investigated [e.g, Kennel and Petschek, 1966;Young et al, 1981;Rauch and Roux, 1982;Roux et al, 1982;Anderson et al, 1996;Fraser and Nguyen, 2001;Halford et al, 2010;Pickett et al, 2010;Zhang et al, 2010Zhang et al, , 2011Min et al, 2012;Allen et al, 2013;Lin et al, 2014], plasma properties relevant to EMIC wave excitation have not been fully understood in the magnetosphere due to the wide spatial extent of EMIC wave generation and propagation. For example, Lin et al [2014] recently found that the correlation of positive A hp events, EMIC instability threshold (Σ h > S h ), and EMIC waves observed by the polar-orbiting Cluster spacecraft is low.…”

“…>1 keV H + are freshly injected from the plasma sheet. Their spectra demonstrate "nose-like" structures [e.g., Smith and Hoffman, 1974;Vallat et al, 2007], characteristic of deeper access to the inner magnetosphere at a specific energy. This unique spectral feature often results from comparable westward and eastward components of the E × B and gradient-curvature drifts, resulting in more inward injections of those particular particles.…”

The controlling effect of ion temperature on EMIC wave excitation and scattering

“…Such high f plasma, with plasma density above at least 100 cm −3 only exists just inside the plasmasphere or storm time plume, and while storm time ring current number density can increase up ∼10 cm −3 [Chen et al, 2010], corresponding to a maximum value of h hh ∼ 10% in such high f plasma. Ring current H + anisotropy rarely exceeds 2 [Roux et al, 1982]. Within the reasonable constraints h hh ≤ 10% and A hh ≤ 2, plasma conditions for EMIC waves in resonance with MeV electrons are required to have at least f > ∼25 and A hh > 1.…”

“…This assumption can seriously break down near W He +, where the He + resonant energy is comparable to or less than the He + thermal energy. Earlier observations [Young et al, 1981;Roux et al, 1982] have shown that He + ions can be heated in the direction perpendicular to the magnetic field line up to suprathermal energies (∼a few 100 eV) when EMIC waves were detected, suggesting a net energy transfer from EMIC waves to He + ions. This is consistent with severe cyclotron damping by He + near W He+ found in ray tracing study by Chen et al [2009], showing suppression of the instability just below W He+ (>3.75W O+ ).…”