Ion Acoustic Wave Observations Near Low-Mach Reverse Shocks*

Abstract Interactions between particles and ion acoustic waves (IAWs) are considered a potential energy dissipation mechanism in low-Mach number shocks, particularly in forward shocks. However, their role in reverse shocks remains unclear. Compared with forward shocks, IAW occurrence in 88 reverse shocks (63 reliable shocks) observed by Wind at 1 au (2008–2020) is extremely low; none are detected within the shock ramp, and only 75 IAWs appear in upstream or downstream regions (only one with amplitude >5 mV m −1 ). This scarcity should be attributed to enhanced Landau damping under high-speed solar wind and low electron-to-ion temperature ratios ( T e / T i < 1). The detected IAWs could be excited by a compression-driven fluid instability associated with the shock’s spatial density gradient or the electron heat flux. To quantify the ability of IAWs to contribute to shock dissipation, we found that the anomalous resistivity from IAWs ( η IA ~ 9.5 × 10 −3 –46 Ω m) is 1–2 orders of magnitude smaller than that reported for forward shocks. Corresponding dissipation ratios ( R Ψ and Y Ψ ) remain well below 1 for all events, indicating that IAW dissipation falls far short of the macroscopic entropy-related energy dissipation requirement. However, nonlinear wave–particle interactions can enhance η IA by 2–3 orders of magnitude, suggesting that IAWs may contribute intermittently but cannot dominate the dissipation in low-Mach number reverse shocks. These results imply that reverse shocks rely on a dissipation pathway fundamentally different from that of forward shocks, one not primarily mediated by IAWs.