Testing linear theory of EMIC waves in the inner magnetosphere: Cluster observations

Abstract

In this paper, we test whether time periods with hot proton temperature anisotropy are associated with electromagnetic ion cyclotron (EMIC) waves and whether the plasma conditions during the observed waves satisfy the linear theory threshold condition. We identify 865 events observed by the Composition Distribution Function instrument onboard Cluster spacecraft 4 during 1 January 2001 to 1 January 2011 that exhibit a positive temperature anisotropy (A hp = T ? h /T k h À 1) in the 10-40 keV protons. The events occur over an L range from 4 to 10 in all magnetic local times and at magnetic latitudes (MLATs) within ±50°. Of these hot proton temperature anisotropy (HPTA) events, only 68 events have electromagnetic ion cyclotron (EMIC) waves. In these 68 HPTA events, for those at 3.8<L ≤ 5 and |MLAT| ≤ 10°, the EMIC waves with powers >1.0 nT 2 /Hz mainly appear in the region with f EMIC /f H,eq < 0.8. Two stop bands are present, one near the region with f EMIC /f H, eq ≈ 0.33, the other in the region with 0.8 < f EMIC /f H,eq < 0.9. Most of the EMIC waves in the He, H, and >H bands satisfy A hp /(A hp + 1) > f EMIC /f H,lo , A hp /(A hp + 1) > 0.45 Â f EMIC /f H,lo , and A hp /(A hp + 1) < 0.45 Â f EMIC /f H,lo . f EMIC , f H,eq , and f H,lo are the EMIC wave frequency, the magnetic equatorial, and the local proton gyrofrequencies. We also find that the EMIC waves predominantly occur with A hp > 0.25. By testing a threshold equation for the EMIC instability based on linear theory, we find that for EMIC waves with |MLAT| ≤ 10°in the He, H, and >H bands, the percentages that satisfy the predicted conditions for wave growth by the threshold equation are 15.2%, 24.6%, and 25.6%. For the EMIC waves with |MLAT| > 10°the percentages that satisfy the wave growth predicted conditions are only 2.8%, 2.6%, and 0.0%. Finally, possible reasons for the low forecast accuracies of EMIC waves are suggested.

A statistical study of EMIC waves observed by Cluster: 2. Associated plasma conditions

“…Hot H + thermal anisotropy in the inner magnetosphere (~5 R E ) increases from dusk, through noon, to dawn. This inner magnetosphere distribution agrees well with previously published statistical results [ Denton et al ., ; Zhang et al ., ; Blum et al ., ; Min et al ., ; Lin et al ., ]. However, there is no significant MLT variation observed in the outer magnetosphere (>10 R E ), other than a slight enhancement around noon.…”

“…The dusk sector (Figure c) peak in hot H + anisotropy is observed in the outer magnetosphere, near 30° |MLAT|, similar to the dawn sector. Lin et al [] also observed that the tendency is for the inner magnetosphere temperature anisotropy to become stronger from dusk, through noon, to dawn. While Lin et al [] observed that the peak in hot H + temperature anisotropy is becoming broader in |MLAT| along with the increasing value of anisotropy, the observed wave‐associated peak presented here seems to be slightly higher for noon than dawn and then considerably lower for dusk.…”

“…Lin et al [] also observed that the tendency is for the inner magnetosphere temperature anisotropy to become stronger from dusk, through noon, to dawn. While Lin et al [] observed that the peak in hot H + temperature anisotropy is becoming broader in |MLAT| along with the increasing value of anisotropy, the observed wave‐associated peak presented here seems to be slightly higher for noon than dawn and then considerably lower for dusk. However, it is unclear if this discrepancy is simply due to not enough wave observations for lower L shells in this study.…”

“…This thermal anisotropy acts as the free‐energy provider for EMIC waves [ Cornwall , ; Rauch and Roux , ]. Another important plasma property is the total density of ions (equal to electron density), as this plasma parameter is related to the threshold of EMIC instabilities and thus the wave growth rate [ Cornwall , ; Young et al ., ; Gary and Lee , ; Gary et al ., ; Blum et al ., ; Lin et al ., ].…”

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. 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.…”