Dynamics and Drivers of Multi-Scale Storm-Time Ionospheric Density Structures

Abstract

Large disturbances in the solar wind, such as Coronal Mass Ejections, can trigger the most significant geomagnetic activities near the Earth, i.e., storms. In the ionosphere, various electron density structures can form during storms, which are usually accompanied by abrupt electron density gradients that can significantly impact the propagation of radio signals going through or reflected by the ionosphere. The spatial scales of these storm-time electron density structures span from thousands of km to several km or less.

Formation and dynamic evolution of these ionospheric density structures are manifests of the complex Magnetosphere-Ionosphere-Thermosphere (MIT) coupling processes during storms. Using a combination of multi-instrument observations and numerical simulations, we performed a comprehensive study investigating various aspects of these dynamics and their drivers. The findings of this study contribute to our systematic understanding of the storm-time MIT coupling processes and could assist future missions targeting these density structures.

In this research, we studied the formation processes of two density structures that often occur at mid and high latitudes during storms: Storm Enhanced Density (SEDs) and polar cap patches. SEDs are electron density enhancements at mid-latitudes and are often suggested as the main plasma sources for the patches in the polar cap region when they extend to high latitudes. Several mechanisms have been proposed in the past to explain the segmentation of SEDs into patches. Using state-of-the-art simulation tools, we proposed a new segmentation mechanism associated with boundary flows between the Region-1 and Region-2 Field Aligned Currents (FACs) when the partial ring current drifts towards the dayside. No sudden external interplanetary magnetic field variations or transient reconnection are required in this mechanism.

Besides the boundary flows, other fast convection flow structures and their temporal variations, such as SAPS, have also been suggested to be important for ionosphere density structuring processes. Using a comprehensive ground- and space-based instrument suite, we performed a detailed analysis of the formation and temporal evolution of SubAuroral Polarization Streams (SAPS) during a moderate storm. We found that the SAPS electric field formed before a substorm onset, and was accompanied by a local magnetic field dip and a dispersionless ion injection from the flank side. In the conjugate ionosphere, the SAPS developed near the equatorward boundary of the Harang reversal. The observed complex signatures in the coupled magnetosphere and ionosphere can be explained by a two-loop current wedge generated by the enhanced plasma pressure gradient and the diamagnetic effect of the structured ring current following the particle injection.

One recently discovered aspect of the storm-time ionospheric responses is the hemispheric asymmetry. We performed a detailed case study of the drivers of the hemispheric asymmetry in electron density observed during the storm recovery phase. The asymmetries were observed simultaneously in the dawn and afternoon sectors and had opposite polarities. They were suggested to be driven by a combination of the hemispheric asymmetries in the thermospheric composition, vertical drifts, and Travelling Ionospheric Disturbance (TID) activity.