LOW-FREQUENCY MAGNETIC FLUCTUATION SPECTRA IN THE MAGNETOSHEATH AND PLASMA DEPLETION LAYER

Recent observations have delineated several different kinds of enhance d magnetic fluctuation spectra below the proton cyclotron frequency in the terrestrial magnetosheath. A model is presented that represents t he variation of plasma parameters across the plasma depletion layer an d into the magnetosheath proper. Using this model, we find that many o f the properties of the observed spectra follow directly from the pred ictions of linear Vlasov instability theory. The observed progression of spectral features is a natural progression from mirror mode to merg ed (in frequency range) proton and He2+ cyclotron modes to bifurcated (in frequency range) cyclotron modes as plasma convects earthward in t he magnetosheath. The necessary change in dispersion surface topology from separated proton and He2+ cyclotron surfaces at low beta to merge d surfaces at high beta is described.