Unusual wave activity near interplanetary shocks

It has been reported that quasiparallel (Q(parallel to)) shocks are generally characterized by hours of upstream ion-acoustic-like wave activity and that quasiperpendicular (Q(perpendicular to)) shocks show only minimal upstream wave activity. However, many exceptions to these trends have been observed by the ULYSSES Unified Radio and Plasma wave (URAP) Experiment near interplanetary shocks. These exceptions include: (1) Q(perpendicular to) shocks with extended periods of high-frequency ion-acoustic-like wave activity in the upstream regions and (2) Q(parallel to) shocks with minimal wave activity upstream. Moreover, the peak amplitudes of the upstream, transition and downstream ion-acoustic-like waves are not highly correlated with the angle between the magnetic field and the shock normal, theta(Bn). These observations do not indicate any systematic difference in the ion-acoustic-like wave activity associated with Q(perpendicular to) and Q(parallel to) shocks. The implications of the present observations for the generally accepted scenarios of upstream wave and particle phenomena and the theories of the origin of diffuse ion distributions are discussed.