Solar Wind Coupling and Magnetospheric Substorms

Abstract:

The concept of the auroral substorm revolutionized the study of geomagnetic activity. Auroral features are not fixed in local time and latitude but instead grow and decay in a systematic pattern. The concept quickly evolved to include other phenomena such as electron precipitation and magnetic pulsations which implied that the substorm had a global scale, the magnetospheric substorm. The original description of the substorm as quiet, expansion, and recovery was modified to include a growth phase. Observations at synchronous orbit showed that auroral activity is associated with a rapid field rotation called dipolarization. This rotations is caused by diversion of tail current in a system called the substorm current wedge. Satellite observation quickly led to the development of the near Earth neutral line model where differences in the rate of magnetic reconnection on the dayside and in the tail led to the systematic changes observed in the substorm. The idea that geomagnetic activity and the substorm was actually caused by reconnection was supported by the observation of a semiannual variation in geomagnetic activity that could be explained by geometry of the Earth and solar wind. Essential to the neutral line model is the idea that magnetic flux is loaded into the tail in the growth phase and unloaded in the expansion phase. An alternative view is that activity is directly driven by the solar wind. Studies of coupling of the solar wind electric field to activity indices supported the idea that a substantial fraction of the variance can be explained by direct driving but unloading of magnetic energy also contributes. Multiple spacecraft observations in the tail led to numerous changes in the original neutral line model. It has been found that multiple bursts of reconnection and fast earthward flow lead to the accumulation of magnetic flux close to the Earth, the development of the current wedge, and the onset of the auroral expansion.