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 three 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 bi‐Maxwellian distribution functions. Results and trends indicated by the theory are found to be in qualitative agreement with the data. The observations show (1) at the inner PDL position the spectral PSD weakens as P dyn decreases, and (2) 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, as a consequence of smaller P dyn , near the magnetopause our temperature anisotropies A p are at the lower end of values characterizing bifurcated spectra in AMPTE/CCE.