A multipoint study of a substorm occurring on 7 December, 1992, and its theoretical implications

On 7 December 1992, a moderate substorm was observed by a variety of satell ites and ground-based instruments. Ionospheric flows were monitored near du sk by the Goose Bay HF radar and near midnight by the EISCAT radar. The obs erved flows are compared here with magnetometer observations by the IMAGE a rray in Scandinavia and the two Greenland chains, the auroral distribution observed by Freja and the substorm cycle observations by the SABRE radar, t he SAMNET magnetometer array and LANL geosynchronous satellites. Data from Galileo Earth-encounter II are used to estimate the IMF B-z component. The data presented show that the substorm onset electrojet at midnight was conf ined to closed field lines equatorward of the preexisting convection revers al boundaries observed in the dusk and midnight regions. No evidence of sub stantial closure of open flux was detected following this substorm onset. I ndeed the convection reversal boundary on the duskside continued to expand equatorward after onset due to the continued presence of strong southward I MF, such that growth and expansion phase features were simultaneously prese nt. Clear indications of closure of open flux were not observed until a sub sequent substorm intensification 25 min after the initial onset. After this time, the substorm auroral bulge in the nightside hours propagated well po leward of the pre-existing convection reversal boundary, and strong flow pe rturbations were observed by the Goose Bay radar, indicative of flows drive n by reconnection in the tail.