RESPONSE OF EQUATORIAL IONOSPHERE TO EPISODES OF ASYMMETRIC RING CURRENT ACTIVITY

We present the characteristics of the response of equatorial ionospher ic zonal electric field and F-region plasma density to the asymmetric ring current intensifications that occurred in succession on 16 Decemb er 1991, corresponding to the STEP/EITS-2 campaign period. The study i s based on high-time-resolution (1-min) data of asymmetic ring current indices, ASY(H/D) and F-region vertical plasma drift, V-z measurement s at Kodaikanal (10.25 degrees N; 77.5 degrees E; dip 4 degrees), Indi a and quarter-hourly ionosonde data of Fortaleza (4 degrees S; 322 deg rees E; dip -9 degrees), Brazil. It is shown that short-lived disturba nces in F-layer vertical plasma drift, V-z and height (h'F/hpF2) indic ative of westward and eastward electric fields prevail simultaneously in the dusk (18-21 LT) and predawn (02-05 LT) sectors, respectively, i n association with the decay phase of asymmetic ring current events. E lectric fields of opposite polarity do also seem to manifest at these local times. particularly in the early-morning sector in conjunction w ith the intensification of the asymmetric ring current. At a given loc ation, electric field disturbances associated with individual asymmetr ic ring current events are thus bipolar in nature, with fields of oppo site polarity during the growth and decay phases. The nature and polar ity structure of the observed electric field disturbances are in agree ment with the theoretical/model predictions of prompt penetration of h igh-latitude electric fields to the equatorial region.