Penetration of polar electric fields to the nightside dip equator at times of geomagnetic sudden commencements

Measurements of Doppler Velocity, VD of F - region reflections at normal incidence over Kodaikanal (dip 3°N, 77° 28′E) are used to study the nature of perturbations in F - region vertical plasma drift, Vz 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* type at middle- and high- latitude stations in the afternoon sector. At Kodaikanal, which is on the nightside, the sc of January 1, 1992, is characterized by a double-step main impulse (MI) in H - component (the structure of the sc on July 8 could not be ascertained from normal run magnetograms because of the large amplitude and very small rise time of the sc). It is found that the usual downward motion of F - region plasma during the premidnight hours at Kodaikanal suddenly ceased (and even reversed to upward in one event) for ∼1 min coincident with the preliminary impulse (PI) and was immediately enhanced in association with the MI of the sc*. This pattern which is consistently seen in the two events implies that an eastward electric field prevails near the nightside dip equator at the time of the first impulse of double-step MI there and the PI of sc* at high latitudes. Our Doppler velocity observations constitute the first and direct experimental evidence of vertical plasma motions due to the sc - associated electric fields in the nighttime dip equatorial ionosphere. They substantiate the view based on theory (Kikuchi and Araki, 1979) and ground-based magnetic observations (Araki et al, 1985) that the dusk-to-dawn electric field imposed on the polar ionosphere with the onset of PI of sc* is instantaneously transmitted to the dip equator on the nightside as on the dayside.