Spectral analysis of magnetohydrodynamic fluctuations near interplanetary shocks

Abstract

Evidence for the presence of relatively large amplitude right-hand elliptically polarized MHD waves upstream and downstream of quasi-parallel (θBn < 45°) forward and reverse interplanetary shocks is presented. The mode is observed with frequencies in the range of 0.025–0.17 Hz (in the spacecraft frame) and propagates along the magnetic field. The MHD modes have rest frame frequencies in the range 3.8–31 mHz, with typical wavelengths about 1.5×10⁸ to 4.7×10⁸ cm. The magnetic field power spectrum in the vicinity of these interplanetary shocks is much steeper than f−5/3 at high frequencies. The observed spectra have a high frequency dependence of f⁻².⁵ to f⁻⁴. A peculiar feature of the fast mode identification in one event is the large correlation observed between |B| and proton density ρ for field-aligned propagation. This appears to be a nonlinear effect, second order in the wave amplitude. The properties of the MHD waves can be understood in terms of the electromagnetic ion-cyclotron instability. Both resonant and nonresonant interactions need be considered to account for the polarization and spectral content of the observed fluctuations. In association with one event, a distinct suprathermal component was observed in the solar wind proton distribution. The parameters of this distribution are adequate to drive a resonant instability with a maximum growth rate that coincides with the observed range of frequencies.