A Statistical Study of the Spatial Scale of Substorm Current Wedge and Its Associated Auroral Images and Near Geosynchronous Observations

It is difficult to determine the spatial extent of current disruption associated with substorm current wedge. The difficulty is due to the fact that satellites are rarely positioned properly to distinguish azimuthal from radial structures. In order to accurately determine the dipolarization onset time, it is necessary to know where the satellite is positioned relative to the substorm current wedge. The accurate substorm timing is one of the main goals of the recently launched THEMIS mission. Several methods can help us to determine the central meridian and azimuthal width of substorm current wedge. The easiest one uses auroral images. The bright surge like western termination of the bulge is associated with the upward field-aligned current. The comparison of auroral images from the IMAGE and POLAR UVI satellites can give us a crude estimation of where the substorm current wedge is located relative to the GOES satellites. However, a more convenient method is to invert ground magnetometer data to obtain the parameters of a model current system that gives the best fit to the observed magnetic perturbations. The model currents include three components: the substorm current wedge, symmetric ring current and partial ring current. This inversion method is capable of obtaining the intensity as well as the longitudinal boundaries of the substorm current wedge with accuracy better than 0.5 hour MLT. There are more than 40 mid-latitude stations available during fall 2001 to 2006. MEASURE, SMALL, 210 MM and INTERMAGNET are arrays with many stations. We have established a substorm onset list from 2001 to 2007. This substorm list will be used as a basis for the development of the inversion code to determine the central meridian and width of substorm current wedges. The estimated central meridian and azimuthal width of substorm current wedge will be compared with auroral images and near geosynchronous observations (GOES and POLAR). The accuracy and relative timing between these observations will be examined.