No 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.

Analyzer on board the Cluster spacecraft observed a prolonged He + energization event from 06:00 to 10:30 UT. In this paper, we perform a case study of the event by using in situ plasma and magnetic field measurements from the Cluster and Double Star Program (DSP) Tan Ce 1 (TC1) spacecraft. In the event, the He + ions were energized up to 1 keV. The He + ion heating was associated with three consecutive EMIC waves, which occurred near the dusk-side magnetospheric flank, L = 13.1-14.5, a region where EMIC wave activity has not been reported before. As a result, the observed wave frequencies, as low as 0.03 Hz, are far lower than those of typical EMIC waves, i.e., 0.1-5 Hz. It is found that the first wave had already propagated for some time and exhibited a large spatial extent, while the latter two were newly generated at the expense of anisotropic, energetic (>1 keV) protons and had sharper spatial boundaries. Unlike the patchy wave activity, the He + energization region displayed a continuous spatial distribution and corresponded to a region of enhanced cold ion density. The reason for the presence of the anisotropic protons and cold density in this region is most likely the quiet geomagnetic conditions, which allow the proton anisotropy to develop and the plasmaspheric plume to expand into such high L-values. A discrepancy, found in the comparison of particle and wave observations with linear theory, suggests that the theory should include He + and/or nonlinear effects.

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 3.81.0 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.

Time periods in which heavy ions dominate over H+ in the energy range of 1–40 keV were observed by the Cluster Ion Spectrometry (CIS)/COmposition DIstribution Function (CODIF) instrument onboard Cluster Spacecraft 4 at L values less than 4. The characteristic feature is a narrow flux peak at around 10 keV that extends into low L values, with He+ and/or O+ dominating. In the present work we perform a statistical study of these events and examine their temporal occurrence and spatial distribution. The observed features, both the narrow energy range and the heavy‐ion dominance, can be interpreted using a model of ion drift from the plasma sheet, subject to charge exchange losses. The narrow energy range corresponds to the only energy range that has direct drift access from the plasma sheet during quiet times. The drift time to these locations from the plasma sheet is > 30 h, so that charge exchange has a significant impact on the population. We show that a simple drift/loss model can explain the dependence on L shell and MLT of these heavy‐ion‐dominant time periods.