A statistical study of EMIC wave-associated He⁺energization in the outer magnetosphere: Cluster/CODIF observations

Abstract

, the Composition and Distribution Function (CODIF) Analyzer on board Cluster observed 41 prolonged He + energization events, lasting for 1.10-4.97 h, on average, 3.18 ± 0.91 h. These He + heating events occurred predominantly at low/middle magnetic latitudes (MLAT = −4.3°-51.7°) in the afternoon sector in the outer magnetosphere (L = 7.9-14.6). During the events, the He + ions resonantly interacted with electromagnetic ion cyclotron (EMIC) waves and were perpendicularly energized to energies up to 1 keV. Their contribution to total ion density in the energy range of 0.04-1 keV was elevated on average up to 51%. A superposed epoch analysis of the plasma data measured by Cluster during these events indicates the presence of the two EMIC wave-controlling factors: hot anisotropic H + (the wave free-energy provider) and cold dense plasma (the wave generation catalyst). In addition, it is common in the events that the density of the energetic H + is elevated and electron plasma/gyrofrequency ratio (f pe /f ce ) reaches values higher than 10. Quiet solar wind and geomagnetic activity appear to be favorable conditions for the generation of the EMIC waves and thus the resultant He + energization in the outer-magnetospheric region. The reason is that, under quiet solar wind and geomagnetic conditions, an overlap of hot anisotropic H + from the plasma sheet and cold dense plasma from a plasmaspheric plume or plume-like region could exist in the afternoon sector of the outer magnetosphere.

Global magnetospheric response to an interplanetary shock: THEMIS observations

“…Before the shock passage at 02:25 UT, the total density n pot was 10 cm −3 and the ESA electron density was 1 cm −3 (not shown). Therefore, the density of the cold population is 9 cm −3 , indicating that THEMIS A most probably observed a plasmaspheric plume (e.g., Goldstein et al, 2004;Zhang et al, 2011). The density increased to 30 cm −3 at 02:26 UT and 100 cm −3 at 02:29 UT.…”

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

“…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.…”

“…EMIC waves are preferentially generated in regions where hot anisotropic H + and cold dense ion populations spatially overlap [e.g., Jordanova et al, 2001;Pickett et al, 2010;Zhang et al, 2011]. In the terrestrial magnetosphere, these ion populations can commonly be found where the ring current overlaps the plasmasphere or plasmaspheric plumes [Gurgiolo et al, 2005;Thorne, 2010].…”

Alfvén‐cyclotron instability with singly ionized helium: Linear theory

“…Between these two branches is a frequency stop band in which no Alfvénic fluctuations can grow [ Gomberoff and Neira , 1983; Kozyra et al , 1984]. This stop band is often observed in EMIC spectra from the outer magnetosphere [e.g., Young et al , 1981; Roux et al , 1982; Zhang et al , 2010, 2011]. Second, cool helium ions also permit the lower frequency branch to become unstable at wavelengths shorter than those which would be unstable in an electron‐proton plasma.…”