Additional stratifications in the equatorial F region at dawn and dusk during geomagnetic storms: role of electrodynamics

Abstract

The role of electrodynamics in producing additional stratifications in the equatorial F region (F₃ layer) at dawn and dusk during geomagnetic storms is discussed. Two cases of F₃ layer at dawn (0600-0730 LT on 5 October 2000 and 8 December 2000) and one case of F₃ layer at dusk (1600-1730 LT on 5 October 2000) are observed, for the first time, by the digital ionosonde at the equatorial station Trivandrum (8.5° N; 77° E; dip ~0.5° N) in India. The unusual F₃ layers occurred during the geomagnetic storms and are associated with southward turning of interplanetary magnetic field B_z , suggesting that eastward prompt penetration electric field could be the main cause of the F₃ layers. The dawn F₃ layer on 5 October is modeled using the Sheffield University Plasmasphere-Ionosphere Model by using the E × B drift estimated from the real height variation of the ionospheric peak during the morning period. The model qualitatively reproduces the dawn F₃layer. While the existing F₂ layer rapidly drifts upward and forms the F₃ layer and topside ledge, a new layer forming at lower heights develops into the normal F₂ layer.