Equatorward shift of the cusp during magnetospheric substorms

The equatorward shift of the midday part of the auroral oval (the cusp region) during the expansion phase of magnetospheric substorms is reexamined. A magnetospheric model developed by Akasofu and Corrick (1980) is used, which is constructed by using the earth's dipole field, an image dipole field of the earth and 40 bisected circular loops. A new dayside current system represented by six current loops separated by 15 deg in longitude, each carrying 100 KA, is added to this model. For a total current of .6 MA, the latitude of the cusp shifts equatorward by 1.8 deg; for a current of 1.8 MA, the cusp location shifts by more than 5 deg. Thus, the magnetic field of an enhanced dynamo current is the dayside boundary layer and of the connected circuit can account for the observed equatorward shift of the cusp region.