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) a ctivity (growth and damping) in the terrestrial plasma depletion layer (PDL ) under typical solar wind conditions. Aside from the ct particles, all par ameters 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 s uperposed epoch analysis technique [Phan et al., 1994]. Three locations wit hin the PDL are specified using minutes before the key time identifying the magnetopause crossing in the superposed epoch analysis (0, -5, -10 min). O ur work thus characterizes average properties of EICWs along radial profile s in the PDL in the magnetic latitude and local time ranges of +/-30 degree s and 0800 to 1600 hours, respectively. In order to study the situation in the subsolar region more closely, we calculate also with PDL parameters acq uired by AMPTE/IRM during the pass nearest to local noon (October 24, 1985) . For alpha particle parameters we take reasonable estimates but also study the effect of varying the alpha particle temperature anisotropy. The influ ence of a small alpha particle-proton relative drift is also included. At t he magnetopause we find one peak of activity in the frequency range from 0. 2 to 0.3 Omega(p), (the proton cyclotron frequency), which is thus below th e resonance frequency of the alpha particles (0.5 Omega p) At locations 10 min and 5 min prior to the magnetopause crossing, the activity spectrum bif urcates, with both peaks below the alpha resonance and separated by an abso rption band. Our explanation of this result is that as we move away from th e magnetopause, the increasing plasma beta enables the protons to overcome the a absorption. With a relative alpha particle-proton drift of 10% of the local Alfven speed, the absorption band is removed, but all excited freque ncies are still less than 0.5 Rp The absorption band may also be removed by a high alpha particle thermal anisotropy coupled with a low alpha particle beta along the magnetic field. On October 24, 1985, the PDL was characteri zed by a wider variation of beta and a larger proton temperature anisotropy . Under these conditions a second EICW activity peak appears between 0.5 an d 0.6 Omega(p) at the two locations away from the magnetopause, in addition to the ct peak mentioned above, which we ascribe to the larger anisotropy of the protons. Again, a small ct particle drift can remove the activity mi nimum. With the stagnation line flow pattern in the PDL found by AMPTE/IRM on these passes, we hypothesize that EICWs excited near noon subsequently t ravel along the magnetic field, bringing EICW activity to high latitudes. H owever, EICWs generated away from noon will be damped out while being carri ed to the magnetopause flanks.