Toward a synthesis of equatorial spread F onset and suppression during geomagnetic storms

Abstract

We present initial steps toward unifying our understanding of storm time equatorial spread F (ESF) by searching for the common elements in past case studies and statistical occurrence patterns. We show that the development (or inhibition) of equatorial irregularities during magnetically active periods can be understood using the AE‐parameterized Fejer‐Scherliess model for disturbance vertical drifts versus storm time and local time. This model takes into account the different sources of perturbation electric fields (magnetospheric and ionospheric dynamos) that ultimately drive the equatorial vertical drifts, showing prompt and delayed effects in the premidnight sector (where both generation and suppression can occur), as well as in the postmidnight period where generation dominates. The postsunset period exhibits the greatest variability for storm time ESF versus longitude, and thus we demonstrate the Fejer‐Scherliess model's applicability in a test case (6 April 2000) that had an AE pattern compatible with their parameterization scheme. The model successfully accounts for the pronounced longitude confinement in the observed postsunset ESF patterns. Finally, we move beyond the empirically derived relationships between geomagnetic indices and the occurrence of ESF (Aarons, 1991) into a framework of true solar‐terrestrial parameters that drive such effects. Additional case studies taken from the published literature are then used to show a consistent linkage between postsunset ESF onset and the interplanetary electric field (IEF) Esw. While AE, Dst, Kp and Dst/dt indices were used in earlier studies to determine the dusk‐longitude sector of disturbance electric fields, here we attribute to the IEF the main role in the determination of this longitude sector.