EXCITATION OF LEFT-HAND POLARIZED NONLINEAR MHD WAVES AT COMET GRIGG-SKJELLERUP

We present a self-consistent theory and the results of numerical simul ations describing the excitation and nonlinear evolution of left-hand polarized Alfven waves in the solar wind near comet P/Grigg-Skjellerup . During the Giotto spacecraft encounter with this comet, the interpla netary magnetic field was almost perpendicular to the solar wind flow, so the newly created (by photoionization) cometary ions excited left- hand polarized waves through normal Doppler resonance. In our calculat ions we take into account two types of nonlinearity (1) the nonlineari ty of the cometary ions' motion due to their resonant interaction with excited waves (i.e., trapping and bounce oscillations 1, and (2) the nonlinearity of the Solar wind particle motion that leads to the steep ening of the wave profile. Results of the study are in reasonable agre ement with observations. This analysis may also be relevant to the rel ated problem of nonlinear wave excitation by reflected protons near pl anetary bow-shocks.