Ion‐Acoustic Waves Associated With Interplanetary Shocks
J. J. Boldú, D. B. Graham, M. Morooka, M. André, Y. V. Khotyaintsev, A. P. Dimmock, David Píša, J. Souček, M. Maksimovic, P. Louarn, A. Fedorov, Georgios Nicolaou, C. J. Owen
Abstract
Abstract Ion‐acoustic waves (IAWs) commonly occur near interplanetary (IP) shocks. These waves are important because of their potential role in the dissipation required for collisionless shocks to exist. We study IAW occurrence statistically at different heliocentric distances using Solar Orbiter to identify the processes responsible for IAW generation near IP shocks. We show that close to IP shocks the occurrence rate of IAW increases and peaks at the ramp. In the upstream region, the IAW activity is highly variable among different shocks and increases with decreasing distance from the Sun. We show that the observed currents near IP shocks are insufficient to reach the threshold for the current‐driven instability. We argue that two‐stream proton distributions and suprathermal electrons are likely sources of the waves.