The Thermalization of Oxygen Ions and Protons at Shocks Associated with CIRs

Abstract

Using analytical theory and data from the Plasma and Suprathermal Ion Composition (PLASTIC) investigation aboard NASA’s Solar Terrestrial Relations Observatory (STEREO), we investigate the thermalization of protons and oxygen ions passing through a shock associated with a corotating interaction region (CIR). The theoretical prediction for the thermalization of protons is based on a simple model of charged particles crossing through an electrostatic shock potential. The oxygen ions just downstream have a different speed relative to the bulk downstream plasma, and the oxygen ions gyrate downstream of the shock. Their thermal speed is the initial gyrospeed immediately after passing through the shock potential; later on part of the initial energy is lost to waves as a result of an Alfvén Ion‐Cyclotron instability. We choose time periods when STEREO AHEAD (STEREO‐A) crossed two low‐Mach‐number shocks. These shocks are chosen such that thermalization contributed by reflected protons and the waves generated by the reflected protons are likely negligible. The predicted proton and oxygen thermal speeds match the observations very well.