Variable Responses of Equatorial Ionosphere During Undershielding and Overshielding Conditions

Abstract

AbstractThis study tests the common assumption that the whole of the equatorial ionospheric F region responds uniformly to both undershielding prompt penetration and overshielding fields. Few recent studies have reported that the bottomside response to geomagnetic storms can differ significantly from that of the topside ionosphere. In this investigation, using vertical drift measurements from Jicamarca incoherent scatter radar in the altitude range of about 200 to 900 km, F region response to the space weather event on 9 November 2004 is studied. It is found that the F region showed large altitudinal variations in vertical drifts only under the influence of overshielding field. Such variations were negligible during undershielding prompt penetration conditions. In a first of its kind of study, direct observations of continuous altitudinal variations in vertical plasma drifts have been used to understand the response of F region ionosphere. This differential response to external drivers has a direct impact on the plasma transportations in the ionosphere and thus on estimating satellite drags. The observations are compared with the widely used ionosphere/thermosphere models: Thermosphere Ionosphere Electrodynamics General Circulation Model and Utah State University Global Assimilation of Ionospheric Measurements. It is found that contrary to observations, the Thermosphere Ionosphere Electrodynamics General Circulation Model shows very small altitudinal gradients in vertical drifts during both undershielding and overshielding conditions. Also, significant difference in the electron transportation especially at the bottomside between two models is adverted. Based on the observations, possible underlying physical processes of asymmetric interhemispheric potentials, different scale sizes of R1 and R2 current systems and different propagation modes for penetrating undershielding and overshielding fields are proposed.