Are high‐latitude forward‐reverse shock pairs driven by CME overexpansion?

Abstract

During its passage through the high‐latitude heliosphere, Ulysses observed interplanetary coronal mass ejections (ICMEs) bounded by shocks. These forward‐reverse shock pairs have only been observed at high latitude in the fast solar wind. It has been suggested (e.g., Gosling et al., 1995) that these shock pairs are the result of expansion of coronal mass ejections into the ambient solar wind, so‐called “overexpansion”. Here we demonstrate an alternative explanation for forward‐reverse shock pairs by means of a three‐dimensional numerical magnetohydrodynamics (MHD) model of a CME interacting with the solar wind. Our global steady state coronal model possesses fast and slow speed solar wind at high and low latitudes, respectively, reminiscent of near solar minimum conditions. Within this model system, a CME erupts from the coronal streamer belt with an initial speed in excess of 1000 km/s, which naturally drives a forward shock into the ambient solar wind. When the CME is greater than 40 R⊙ from the Sun, we find that a reverse shock forms poleward of the CME as a result of the interaction of the CME with the solar wind. In front of the CME, the slow wind is deflected to higher latitude while behind the CME, fast wind is deflected to low latitude. These deflected streams collide to form a reverse shock. The shock pair formed in this way naturally occurs at high latitude in the fast wind as observed. We will discuss these model results in the context of in situ solar wind data.