Response of equatorial ionosphere to episodes of asymmetric ring current activity

We present the characteristics of the response of equatorial ionospheric zonal electric ®eld and Fregion plasma density to the asymmetric ring current intensi®cations that occurred in succession on 16 December 1991, corresponding to the STEP/EITS-2 campaign period. The study is based on high-timeresolution (1-min) data of asymmetic ring current indices, ASY(H/D) and F-region vertical plasma drift, Vz measurements at Kodaikanal (10.25°N; 77.5°E; dip 4°), India and quarter-hourly ionosonde data of Fortaleza (4°S; 322°E; dip )9°), Brazil. It is shown that short-lived disturbances in F-layer vertical plasma drift, Vz and height (h¢F/hpF2) indicative of westward and eastward electric ®elds prevail simultaneously in the dusk (18±21 LT) and predawn (02±05 LT) sectors, respectively, in association with the decay phase of asymmetic ring current events. Electric ®elds of opposite polarity do also seem to manifest at these local times, particularly in the early-morning sector in conjunction with the intensi®cation of the asymmetric ring current. At a given location, electric ®eld disturbances associated with individual asymmetric ring current events are thus bipolar in nature, with ®elds of opposite polarity during the growth and decay phases. The nature and polarity structure of the observed electric ®eld disturbances are in agreement with the theoretical/model predictions of prompt penetration of high-latitude electric ®elds to the equatorial region.