Electromagnetic Ion Cyclotron Waves in the Subsolar Region Under Normal Dynamic Pressure: Wind Observations and Theory

We analyze observations on electromagnetic ion cyclotron waves (EICWs) in the plasma depletion layer (PDL) made by Wind on 3 inbound passes of the magnetosheath near the stagnation streamline, and model the observations using recent theory (Gnavi et al., 2000). While one pass was under substantially high solar wind dynamic pressure P_dyn ( ∼ 6.4 nPa), two passes were under typical P_dyn at 1 AU ( ∼ 2.2 nPa) which allows us to extend the study of EICWs into this lower pressure regime. Two passes were under steady, and one under time varying, conditions. We subdivide the PDL into inner, middle and outer regions and compute in each the power spectral density (PSD) of the magnetic fluctuations transverse to the background field, using high resolution ( ∼ 11 samples/s) data from the Magnetic Field Investigation (MFI). The theory solves the kinetic dispersion relation in a plasma composed of electrons, protons, and α particles, each species modeled by biMaxwellian distribution functions. Parameters for calculating growth rates are taken from the observations, where available, and from average values quoted in the literature otherwise. Qualitative, and on occasion even quantitative, agreement between theory and data is very good. The observations show (i) at the inner PDL position the spectral PSD weakens as P_dyn decreases, and (ii) the frequency range of emission shifts downward with diminishing P_dyn. Comparing observations with the classification of spectral types in the PDL under compressed conditions proposed by Anderson et al. [1994], we find a preponderance of so-called low and continuous spectra and only marginally bifurcated spectra with activity peaks below and above the alpha-resonance frequency. In general, near the magnetopause our temperature anisotropies A_p are at the lower end of values characterizing bifurcated spectra in AMPTE/CCE. Using bipolytropic laws for the anisotropic magnetosheath, we argue that the effect of a reduction of P_dyn is to lower A_p, thereby weakening the driver of EICWs leading to marginally bifurcated spectra and weaker EICW activity in the PDL under typical conditions. This work is partially supported by NASA grants NAG5-13116, NAG5-11803, and and the Argentinean UBACYT grant X059.