PENETRATION OF POLAR ELECTRIC-FIELDS TO THE NIGHTSIDE DIP EQUATOR AT TIMES OF GEOMAGNETIC SUDDEN COMMENCEMENTS

Measurements of Doppler Velocity, V(D) of F - region reflections at no rmal incidence over Kodaikanal (dip 3-degrees-N, 77-degrees-28'E) are used to study the nature of perturbations in F - region vertical plasm a drift, V(z) associated with the geomagnetic sudden commencements (sc ) on July 8, 1991, and January 1, 1992. Both the events which occurred in a narrow time window, 1630-1700 UT (2200-2230 IST) were of sc typ e at middle- and high- latitude stations in the afternoon sector. At K odaikanal, which is on the nightside, the sc of January 1, 1992, is ch aracterized by a double-step main impulse (MI) in H - component (the s tructure of the sc on July 8 could not be ascertained from normal run magnetograms because of the large amplitude and very small rise time o f the sc). It is found that the usual downward motion of F - region pl asma during the premidnight hours at Kodaikanal suddenly ceased (and e ven reversed to upward in one event) for approximately 1 min coinciden t with the preliminary impulse (PI) and was immediately enhanced in as sociation with the MI of the sc. This pattern which is consistently s een in the two events implies that an eastward electric field prevails near the nightside dip equator at the time of the first impulse of do uble-step MI there and the PI of sc at high latitudes. Our Doppler ve locity observations constitute the first and direct experimental evide nce of vertical plasma motions due to the sc - associated electric fie lds in the nighttime dip equatorial ionosphere. They substantiate the view based on theory (Kikuchi and Araki, 1979) and ground-based magnet ic observations (Araki et al, 1985) that the dusk-to-dawn electric fie ld imposed on the polar ionosphere with the onset of PI of sc is inst antaneously transmitted to the dip equator on the nightside as on the dayside.