Evidence for slow mode MHD turbulence in the solar wind: Post‐slow shock observations at 0.31 AU

Abstract

First observational evidence is presented for low‐frequency slow mode MHD turbulence in the solar wind (ƒ ≤ 5 × 10−3 Hz, δB‐δN coherence greater than 0.6) by employing power and coherence spectral analysis of the Helios high time resolution solar wind plasma and magnetic field observations. This turbulence occurs directly behind an interplanetary slow forward shock wave at 0.31 AU. As the foreshock plasma conditions are such that steepening of slow waves is favored in comparison to Landau damping, the observations presented here do in addition provide unique support to the theoretical results of Hada and Kennel (1985). It is also shown that the turbulence upstream is predominantly Alfvénic (ƒ≲ 10−2 Hz, 10−2 Hz, δB‐δV coherence greater than 0.94) and propagating away from the Sun (shock). Thus we provide an example of an interplanetary slow shock where the turbulence upstream and downstream is completely different in nature. Moreover, the “evolutionary conditions” for slow shocks indicate that the downstream (upstream) turbulence could not have been generated by an interaction and wave mode conversion of the upstream (downstream) turbulence with and by the slow shock, respectively.