Ion acoustic‐like waves observed by Ulysses near interplanetary shock waves in the three‐dimensional heliosphere

Abstract

The Ulysses spacecraft encountered 160 interplanetary shock waves between the time of its launch in October 1990 and the end of 1993 when it reached 48° S heliographic latitude. The onboard radio and plasma wave instrument observed many plasma waves associated with these shocks. Electrostatic waves at frequencies between the ion and electron plasma frequencies are observed intermittently throughout the solar wind but are often strongly enhanced near the shocks. These waves are usually interpreted as ion acoustic waves that have been Doppler shifted to the observed frequencies. They show very diverse characteristics in their amplitude and in their temporal behavior relative to the time of the shock passage at Ulysses. They are most likely to occur within a few minutes of shock passage, but on average they are enhanced above background levels for several hours before and after passage of the shock. The temporal behavior of the probability of occurrence of these waves is roughly symmetric relative to the time of shock passage and furthermore is quite similiar for quasi‐parallel and quasi‐perpendicular shocks and for forward and reverse shocks. These waves are correlated with several of the local plasma parameters measured by the Ulysses plasma instrument, most strongly with Te/Tp, the ratio of electron temperature to proton temperature, and with the electron heat flux. There is no significant correlation of the waves with the upstream plasma β or the shock strength. The similiarity of the quasi‐parallel and quasi‐perpendicular cases implies that the mechanism for wave generation is not sensitive to θBn, the angle between the upstream magnetic field and the shock normal. Examination of the particle distribution function at the shocks indicate that beams in the proton distribution function and enhanced electron heat flux may contribute to driving the waves unstable, particularly when Te/Tp is small.