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

Abstract

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 ∥ h − 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 × f EMIC / f H,lo , and A hp /( A hp + 1) < 0.45 × 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.