Determining the strength of the ring and the magnetopause currents during the initial phase of a geomagnetic storm using cosmic ray data

During a geomagnetic storm the strength of the magnetospheric current systems is strongly increased. In the initial phase of most events, however, the magnetic field at the Earth's equator (as characterized by the Dst index) shows only a relatively small perturbation due to the opposite magnetic effects caused by the magnetopause currents compared to the ring current. Analysis of Dst and of the cosmic ray cutoff rigidity changes at about 55° geomagnetic latitude offers the unique possibility to estimate the intensity of these two current systems separately. The procedure is illustrated for the geomagnetic storm on December 17, 1971.