VARIATIONS IN THE POLAR-CAP AREA DURING INTERVALS OF SUBSTORM ACTIVITY ON 20-21 MARCH 1990 DEDUCED FROM AMIE CONVECTION PATTERNS

The dynamic behaviour of the northern polar cap area is studied employ ing Northern Hemisphere electric potential patterns derived by the Ass imilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. The rate of change in area of the polar cap, which can be defined as the region of magnetospheric field lines open to the interplanetary magnet ic field (IMF), has been calculated during two intervals when the IMF had an approximately constant southward component (1100-2200 UT, 20 Ma rch 1990 and 1300-2100 UT, 21 March 1990). The estimates of the polar cap area are based on the approximation of the polar cap boundary by t he flow reversal boundary. The change in the polar cap area is then co mpared to the predicted expansion rate based on a simple application o f Faraday's Law. Furthermore, timings of magnetospheric substorms are also related to changes in the polar cap area. Once the convection ele ctric field reconfigures following a southward turning of the IMF, the growth rate of the observed polar cap boundary is consistent with tha t predicted by Faraday's Law. A delay of typically 20 min to 50 min is observed between a substorm expansion phase onset and a reduction in the polar cap area. Such a delay is consistent with a synthesis betwee n the near Earth neutral line and current disruption models of magneto spheric substorms in which the dipolarisation in the magnetotail may a ct as a trigger for reconnection. These delays may represent a propaga tion time between near geosynchronous orbit dipolarisation and subsequ ent reconnection further down tail. We estimate, from these delays, th at the neutral X line occurs between similar to 35R(E) and similar to 75R(E) downstream in the tail.