Comment on “Are high-latitude forward-reverse shock pairs driven by overexpansion?” by W. B. Manchester and T. H. Zurbuchen

摘要

Recently, Manchester and Zurbuchen [2006] used results from a global MHD simulation of a coronal mass ejection (CME) propagating in the solar wind to propose a new interpretation for the properties of a set of nine heliospheric disturbances bserved by the Ulysses spacecraft at high heliographic latitudes. These events were observed predominantly, but not exclusively, during the declining phase of the solar cycle. Gosling et al. [1994a, 1994b] originally suggested that such events could be produced by the overexpansion of high pressure (either magnetic, thermal, or some combination thereof) CMEs, leading to their name: ‘‘Overexpanding’’ CMEs. In this scenario (modeled numerically in two dimensions by Riley et al. [1997]) a fast CME erupting at low heliographic latitudes, into a bimodal ambient solar wind, would expand in all dimensions, penetrating into the high-speed wind at high latitudes. At low latitudes, the interplanetary CME (ICME) propagates significantly faster than the ambient solar wind and drives a orward (F) shock ahead of it. At high latitudes, however, the ICME becomes embedded within the fast coronal hole stream. The ejecta’s high internal pressure, relative to the surrounding wind, drives a fast-mode wave away from it in almost all directions, leading to the observed F wave (or shock) ahead of it, and the R wave (or shock) behind it.