Equatorial disturbance dynamo electric field longitudinal structure and spread F: A case study from GUARA/EITS campaigns

Digisondes/ionosondes, an HF Doppler radar and magnetometers were operated in Brazil and India during the September/October 1994 GUARA/EITS campaigns. Analysis of the data for the two disturbed intervals, 2-4 October and 25-27 September provided evidence of a longitudinal structure in the disturbance dynamo (DD) electric field at low latitudes. The DD electric field which is westward in the evening, inhibited the developments of the equatorial prereversal electric field and postsunset ESF, whereas the simultaneous eastward field in the predawn sector, did not lead to ESF, contrary to what is normally expected. This nondevelopment,bf ESF is suggested as evidence of the stabilizing effect of transequatorial neutral winds associated with stormtime circulation. The campaign observations clearly demonstrate that the DD electric fields could often mask the low latitude ionospheric responses to prompt penetration electric fields in the course of sustained substorm activity.