Ionospheric Storms in the Sub-Auroral Ionosphere: Local Time Effects Driven by Electrodynamics

The response of the mid-latitude ionosphere to geomagnetic storms depends upon several pre-storm conditions, the dominant ones being season and local time of the storm commencement (SC). The difference between a site’s geographic and geomagnetic latitudes is also of major importance since it governs the blend of processes linked to solar production and magnetospheric input, respectively. Electrodynamics plays a central role in storm morphologies at sub-auroral sites: it contributes to the positive phase near dusk on the first day or a storm, and to the motions of the trough on subsequent nights, and to the Joule heating that drives the negative phase. To explore hemispheric consistency of ionospheric storms, we identify two key locations that are “geophysically-equivalent” sites and offer the optimal ways to assess magnetosphere-ionosphere coupling. At the longitudes of the dipole tilt, we use ionosonde values of the F2-layer maximum electron density (NmF2) to study geophysical equivalency at Wallops Island (VA) and Hobart (Tasmania), using statistical summaries of 206 events during solar cycle #20. We form average patterns of ΔNmF2 (%) versus local time over 7-day storm periods that are constructed in ways that enhance the portrayal of the characteristic features of the positive and negative phases of ionospheric storms. The results show a remarkable consistency between local time patterns of storm-induced perturbations and the processes that cause them in each hemisphere. Subtle differences are found in the role of electrodynamics for two features of the negative phase: convective motions of the trough and the Joule heating that drives the negative phase.