PROPAGATION CHARACTERISTICS OF WAVES UPSTREAM AND DOWNSTREAM OF QUASI-PARALLEL SHOCKS

The propagation characteristics of waves upstream and downstream of qu asi-parallel shocks are investigated by using two-dimensional hybrid s imulations. At low Alfven Mach numbers M(A) less than or similar to 2, the shock is initially associated with upstream phase-standing whistl ers. At later times, backstreaming ions excite longer-wavelength whist lers via the right-hand resonant ion/ion instability. As anticipated f rom linear theory, these waves are found to propagate along the magnet ic field. Their group velocity is larger than or comparable to the ups tream flow speed, and as a result, the waves remain in the upstream re gion. At higher M(A) greater than or similar to 3 these waves are conv ected back into the shock, causing its re-formation and downstream per turbations. Shock transmitted waves mode-convert into Alfven/ion-cyclo tron waves which have a wavevector along the shock normal (pointing up stream) and convect downstream. The two-dimensional simulation results confirm our earlier suggestion that the upstream waves should be fiel d aligned, and that their convection into the downstream is associated with linear mode conversion into the Alfven/ion-cyclotron branch.