Substorm onset: A switch on the sequence of transport from decreasing entropy to increasing entropy

Abstract

In this study, we propose a scenario about the trigger for substorm onset. In a stable magnetosphere, entropy is an increasing function tailward. However, in the growth phase of a substorm, a later born bubble has lower entropy than earlier born bubbles. When a bubble arrives at its final destination in the near-Earth region, it will spread azimuthally because of its relatively uniform entropy. The magnetic flux tubes of a dying bubble, which cause the most equatorward aurora thin arc, would block the later coming bubble tailward of them, forming an unstable domain. Therefore, an interchange instability develops, which leads to the collapse of the unstable domain, followed by the collapse of the stretched plasma sheet. We regard the substorm onset as a switch on the sequence of transport, i.e., from a decreasing entropy process to an increasing entropy process. We calculated the most unstable growth rates and the wavelengths of instability, and both are in agreement with observations.