Comparison of intense nightside shock induced aurora and substorms activity

Variations of the solar wind dynamic pressure induce perturbation of magnetospheric processes. These perturbations frequently induce dayside enhancements of activity with particular features such as low latitude proton flash, low latitude arcs and aurora propagating eastward and westward from noon to the night sector. In some cases, these shocks may also induce an enhancement of the nightside activity during which the power precipitated in the night sector may reach values as high as observed during substorms. Various studies have shown that high precipitated powers are more likely during period of negative values of the North - South IMF components. Liou et al (2003) have shown that substorm-like activity is not frequent after a shock and they concluded that a shock may not be considered as a substorm trigger. The question addressed in this study is to know up to what point the substorm like nightside activity triggered by a shock is comparable to a classical substorm. For this purpose, we analyze four events presenting nightside activity morphologically similar to substorms and occurring within a short time (less than 20 minutes) 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 motion of boundaries before and after onset and the power precipitated in the nightside region are compared to typical substorms. Except for the absence of southward motion of activity before onset, shock induced substorms appear very similar to isolated substorms. We investigate the ability of a shock to trigger a substorm during periods characterized by particular conditions. We suggest that the sign of B[SUB]z[/SUB] plays an important role as well as the history of solar wind and interplanetary magnetic field and the resulting state of the magnetosphere.