<Review>Substorm: view from global MHD simulation and THEMIS observation

Abstract

In the geospace, a disturbance phenomenon called "substorm" are frequently observed, which can cause a great impact on elements in the humanosphere, such as radiation damage and surface charging on the satellite, health risk to astronauts due to the high-latitude radiation environment, and disasters in power transmission networks. To understand the physics of the substorm should be of prime importance, which helps us (1) to avoid such disasters by forecasting, and (2) to be a breakthrough in systematic understanding of the near-Earth environment from a global perspective, because the substorm is a global phenomenon whose signatures can be traced in most regions of the geospace. Triggering mechanism of the substorm expansion onset is one of the most important issues in the substorm research, which has not yet been exactly settled, though a number of models have been advocated previously. In this paper, I will firstly review the observational discovery of the magnetosphere that is the arena of the substorm phenomena, and then make a review on the triggering mechanism of the substorm onset, finally from a viewpoint of comparison between a global MHD simulation and multiple satellites observation the triggering mechanism will be reconsidered by the formation and evolution of high-pressure region origin from the magnetic reconnection site in the middle magnetotail.