Comparison of intense nightside shock‐induced precipitation and substorm activity

Sudden variations of the solar wind dynamic pressure frequently induce dayside enhancements of auroral activity with features such as high‐latitude arcs, low‐latitude proton flashes, and enhancement of auroral precipitation propagating dawnward and duskward from noon to the night sector. In some cases, these shocks also induce enhanced activity during which the power precipitated into the night sector may reach values as high as observed during substorms. Several studies have shown that the triggering of nightside‐enhanced precipitation is more likely during periods of southward interplanetary magnetic field (IMF) components. Early works showed that substorm‐like activity is not frequent after a shock and suggested that shocks may not be considered as substorm triggers. We examine up to what point substorm‐like nightside activity triggered by a shock is comparable to an isolated substorm. For this purpose, we analyze three events morphologically similar to substorms and occurring within less than 20 min after the arrival of a pressure pulse on the front of the magnetosphere. Different features of these events such as the mean energy of precipitated electrons, the latitudinal motion of boundaries before and after onset, and the power precipitated into the nightside sector are compared with isolated substorms. We conclude that the characteristics of shock‐induced substorms appear very similar to those of isolated substorms. Shocks are able to trigger substorms when they hit an unstable magnetosphere. The interpretation is that the perturbation due to the shock induces a substorm by closure of the plasma sheet magnetic field. For the events presented in this study, the instability result from a period of southward IMF or stretching of the magnetic tail induced by a previous shock.