Detailed Structure of Very High‐β Earth Bow Shock

Abstract

AbstractObservations of Earth's bow shock with high (ratio of thermal to magnetic pressure) are rare. However, such shocks are supposed to be ubiquitous in astrophysical plasmas. We present statistics of several tens crossings with for MMS, Cluster and Geotail, 30 of which have . For the latter subset, most of the crossings reveal upstream structure with the periodically emerging activations, gradually thermalizing the solar wind ion flow. One fortuitous MMS shock event allowed to study this phenomenon with unprecedented details. Each activation, in turn, consists of high‐amplitude magnetic variations with the period about 1 s, coupled with the pulses of the plasma flow. These variations have wavelengths of about 150 km, linear polarization and are almost standing in the plasma rest frame, consistent with the expectation for Weibel mode. All together, the transition interval may last 5–10 min, but corresponds to a proton cyclotron scale in the solar wind, due to very low magnetic field and very slow shock motion. The most likely physical cause of this extended upstream activity is reformation of a supercritical shock.