A sunward propagating fast wave in the magnetosheath observed after the passage of an interplanetary shock

Abstract

The interaction of an interplanetary (IP) shock with the terrestrial magnetosphere causes some noticeable effects on the global scale structure of the magnetosphere itself. One of these effects is given by an earthward motion of the bow shock followed, some minutes later, by a sunward displacement. As demonstrated by past observational studies, in agreement with the theory of the shock‐shock collision, the earthward motion of the bow shock is due directly to the interplanetary shock impact on it. Differently, the origin of the sunward motion of the bow shock is still not well understood. In this regard, on the basis of some issues of the present observational study, we suggest a possible mechanism to account for the aforementioned outward displacement. Our event, observed by the Cluster SC3 spacecraft on day 7 November 2004, is related to a magnetosheath perturbation triggered by an IP shock impact. The main result of the data analysis is the first identification of a reverse (i.e., sunward directed) fast magnetosonic wave just after the IP shock passage. The identification has been performed via a direct comparison of the observations with the MHD equations of nonlinear magnetosonic waves. The reverse fast wave consists of a smooth compression, approximately 2 min long, which is most of the magnetosheath plasma density increase produced by the entire perturbation. We point out that signatures of this reverse wave are observable in two other events as well as in a three‐dimensional MHD simulation reported in the literature on the topic. Moreover, we provide a possible interpretation of this reverse wave in terms of a transient pressure build‐up in the magnetosheath due to the post‐shock reconfiguration of the plasma flow around the magnetopause obstacle.