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

Abstract

The characteristics of the geomagnetic sudden commencement (SC) that occurred at 1211:30 UT on November 18, 1993, following an interval of prolonged geomagnetic quietness are studied using high time resolution data of several magnetometer networks. We present the first results concerning the behavior of the preliminary reverse impulse (PRI) of the H component of the SC near the dip equator from simultaneous observations in different longitude (local time) sectors. It is found that the preliminary reverse impulse appeared only in the prenoon (0912 LT) sector very close to the magnetic equator (dip 0.6°). At locations farther away (dip 6.0°–7.2°) but on the same meridian, the preliminary reverse impulse diminished in amplitude and led to a delayed onset of the main impulse (MI) of the SC. The preliminary reverse impulse is not apparent at locations close to the magnetic equator (dip 1.2°–2.7°) in either the afternoon (1300 LT) or the near‐dusk (1740 LT) sectors. What is seen instead here is an unambiguous reduction in the rate of increase of H component coincident with the preliminary reverse impulse in the forenoon sector. HF Doppler radar measurements of F layer vertical plasma drift close to the magnetic equator (dip 4.9°) near the dusk sector showed the SC‐related disturbance to be a decrease in ambient upward drift with considerable temporal structure, which indicates the imposition of a westward electric field. The preliminary reverse impulse is seen in the subauroral to polar region (MLAT 56.8°–76°) on the afternoon side simultaneous with that near the prenoon dip equator and with a conspicuous increase in amplitude and duration with latitude over MLAT range 66°–76°. Theoretical calculations suggest that the global current system set up by a pair of field‐aligned currents at 80° latitude and shifted to morningside (centers at 1300 and 0300 LT) could, in general, account for the observed behavior of the preliminary reverse impulse, except in the dip equatorial region near dusk.