ZONAL MERIDIONAL WIND AND DISTURBANCE DYNAMO ELECTRIC-FIELD CONTROL OF THE LOW-LATITUDE IONOSPHERE BASED ON THE SUNDIAL/ATLAS-1 CAMPAIGN/

We present an evaluation of global-scale equatorial and low latitude i onospheric disturbances in response to the weak-to-moderate disturbed conditions that marked the SUNDIAL/ATLAS1 twelve-day campaign of Match 22 to April 2, 1992. Ionosonde data from South American and Indian-As ian longitudes are analyzed to examine first the climatological (the a verage) pattern of the critical F region parameters (the layer peak de nsity and height) in comparison with the empirical International Refer ence Ionospheric model, and then to characterize the day-to-day variab ilities aiming at identifying potential causal contributions from eith er disturbance zonal and meridional winds or magnetospheric and distur bance dynamo electric fields. Included in this analysis are data from South American midlatitude locations which are used to determine merid ional winds using an adaptation of the servo analysis technique in the Field Line Integrated Plasma (FLIP) model. We have made an assessment of the causal mechanism of the day-to-day variabilities as arising fr om latitude dependent disturbance meridional winds, and from electric fields produced by disturbance zonal winds and disturbance dynamo. Whi le the contribution from disturbance meridional winds decreases from m iddle to equatorial latitudes, that of the electric fields maximizes : around the equator. In particular, first-time evidence based on ionos onde data is provided for a disturbance dynamo electric field in the e quatorial ionosphere. It is found that there are two time intervals of maximum ionospheric variability resulting from the weak to moderate m agnetospheric disturbance conditions that prevailed during the campaig n: one near the evening/postsunset hours and the other in the postmidn ight-sunrise hours over low latitudes. At midlatitudes a broad maximum of the response occurs from premidnight to morning hours. We provide a comparison of results for the South American and Indian-Asian longit udes and a discussion of the competing roles of the disturbance zonal and meridional winds; and magnetospheric and disturbance dynamo electr ic fields as a function OI latitude.