Configuration of the geomagnetic tail during substorms

Imp 4 vector magnetic field measurements and the geomagnetic AE index are used to study the configuration of the geomagnetic tail within 34 R E and its variations during substorms. The existence of a depressed field magnitude region corresponding to the average position of the plasma sheet is confirmed and the average solar magnetospheric component of the field is found to be northward and independent of distance from the expected position of the neutral sheet for |Z 8 m ≲8 R E . The Z 8 m component measured some distance from the equatorial plane is a measure of flux crossing the neutral sheet and can be studied as a function of time during substorms. Prior to and during the early phases of a substorm the tail field magnitude increases and the northward field component decreases as the plasma sheet becomes thin. Later in the substorm the field magnitude decreases and the northward field component increases as the tail field relaxes to a more dipolar state. The greater number of field lines crossing the neutral sheet after the substorm is in agreement with the predictions of reconnection theories. Enhanced Z 8 m components after substorms and during the rare occurrences of very quiet intervals suggest that many more field lines close near the earth and fewer go into the gomagnetic tail at these times.