Theoretical properties of electromagnetic ion cyclotron waves in the terrestrial, dayside, low-latitude plasma depletion layer under uncompressed magnetosheath conditions


We present a numerical study of electromagnetic ion cyclotron wave (EICW) activity (growth and damping) in the terrestrial plasma depletion layer (PDL) under typical solar wind conditions. Aside from the α particles, all parameters in this study are as measured by AMPTE/IRM during 13 magnetosheath passes under low magnetic shear made in 1984–1985 and calculated with the superposed epoch analysis technique [Phan et al., 1994]. Three locations within the PDL are specified using minutes before the key time identifying the magnetopause crossing in the superposed epoch analysis (O, —5, —10 min). Our work thus characterizes average properties of EICWs along radial profiles in the PDL in the magnetic latitude and local time ranges of ±30° and 0800 to 1600 hours, respectively. In order to study the situation in the subsolar region more closely, we calculate also with PDL parameters acquired by AMPTE/IRM during the pass nearest to local noon (October 24, 1985). For α particle parameters we take reasonable estimates but also study the effect of varying the α particle temperature anisotropy. The influence of a small α particle-proton relative drift is also included. At the magnetopause we find one peak of activity in the frequency range from 0.2 to 0.3 Ωp (the proton cyclotron frequency), which is thus below the resonance frequency of the α particles (0.5 Ωp). At locations 10 min and 5 min prior to the magnetopause crossing, the activity spectrum bifurcates, with both peaks below the a resonance and separated by an absorption band. Our explanation of this result is that as we move away from the magnetopause, the increasing plasma β enables the protons to overcome the α absorption. With a relative α particle-proton drift of 10% of the local Alfvén speed, the absorption band is removed, but all excited frequencies are still less than 0.5 Ωp. The absorption band may also be removed by a high α particle thermal anisotropy coupled with a low α particle β along the magnetic field. On October 24, 1985, the PDL was characterized by a wider variation of β and a larger proton temperature anisotropy. Under these conditions a second EICW activity peak appears between 0.5 and 0.6 Ωp at the two locations away from the magnetopause, in addition to the α peak mentioned above, which we ascribe to the larger anisotropy of the protons. Again, a small α particle drift can remove the activity minimum. With the stagnation line flow pattern in the PDL found by AMPTE/IRM on these passes, we hypothesize that EICWs excited near noon subsequently travel along the magnetic field, bringing EICW activity to high latitudes. However, EICWs generated away from noon will be damped out while being carried to the magnetopause flanks.

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JGR: Space Physics



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