Consequences of imperfect magnetosphere-ionosphere coupling

Imperfect magnetosphere-ionosphere coupling, defined by the existence of an aurora-producing electric potential drop along field lines, provides a theoretical framework for the substorm phenomena. The source of the field-aligned potential is believed to be the enhanced convection electric field in the plasma sheet being partially short-circuited by the conducting ionosphere. The mechanism which maintains the auroral potential and the process which controls its field-aligned scale-length are now reasonably well understood and will be summarized. In addition to causing the brightening of discrete auroras, the imperfect magnetosphere-ionosphere coupling also leads to the Pi2 magnetic pulsations and the westward traveling surge which are manifestations of a substorm. The generation of Pi2 pulsations and the distortion of the convection pattern associated with the westward traveling surge are shown to result from the transient ionospheric response to an enhanced magnetospheric convection. Conductivity enhancement due to auroral electron precipitation is shown to play a crucial role in all aspects of substorm phenomena