Preliminary impulse of the geomagnetic storm sudden commencement of November 18, 1993

The characteristics of the geomagnetic sudden commencement (SC) that occurr ed at 1211:30 UT on November 18, 1993, following an interval of prolonged g eomagnetic quietness are! studied using high time resolution data of severa l magnetometer networks. We present the first results concerning the behavi or of the preliminary reverse impulse (PRI) of the H component of the SC ne ar the dip equator from simultaneous observations in different longitude (l ocal time) sectors. It is found that the preliminary reverse impulse appear ed only in the prenoon (0912 LT) sector very close to the magnetic equator (dip 0.6 degrees). At locations farther away (dip 6.0 degrees -7.2 degrees) but on the same meridian, the preliminary reverse impulse diminished in am plitude and led to a delayed onset of the main impulse BMI) of the SC. The preliminary reverse impulse is not apparent at locations close to the magne tic equator (dip 1.2 degrees -2.7 degrees) in either the afternoon (1300 LT ) or the near-dusk (1740 LT) sectors. What is seen instead here is an unamb iguous reduction in the rate of increase of H component coincident with the preliminary reverse impulse in the forenoon sector. HF Doppler radar measu rements of F layer vertical plasma drift close to the magnetic equator (dip 4.9 degrees) near the dusk sector showed the SC-related disturbance to be a decrease in ambient upward drift with considerable temporal structure, wh ich indicates the imposition of a westward electric field. The preliminary reverse impulse is seen in the subauroral to polar region (MLAT 56.8 degree s -76 degrees) on the afternoon side simultaneous with that near the prenoo n dip equator and with a conspicuous increase in amplitude and duration wit h latitude over MLAT range 66 degrees -76 degrees. Theoretical calculations suggest that the global current system set up by a pair of field-aligned c urrents at 80 degrees latitude and shifted to morningside (centers at 1300 and 0300 LT) could, in general, account for the observed behavior of the pr eliminary reverse impulse, except in the dip equatorial region near dusk.