Timescales of Ionospheric Response during a Geomagnetic Storm: Combining Global MHD Simulations and Observation Data

Geomagnetic storms generate a complex and highly time-dependent response in the ionosphere. Understanding the timescales over which magnetosphere-ionosphere coupling occurs during such conditions is thus crucial in accurate forecasting of severe space weather events. By combining global simulations with satellite and ground observations, we can relate observed response timescales to the time-evolution in the large-scale dynamics of the system.

In this study we use the Gorgon MHD code to simulate the state of the magnetosphere-ionosphere system during a geomagnetic storm commencing on 3^rd May 2014. We analyse the morphology of the ionospheric Region 1 field-aligned current (FAC) during the event, identifying various asymmetries across the polar cap. These are then explained in terms of the global dynamics, such as the change in the location of magnetopause reconnection, the reconfiguration of the magnetotail current sheet and the resulting motion of the open-closed boundary, and how these evolve over different timescales as the storm progresses.

We then compare the simulation results to observations of the same event using FAC data from AMPERE and ground magnetic field data from SuperMAG. These data are cross-correlated at individual coordinate bins to the upstream IMF B_y and B_z, generating spatial maps of the correlation and timescale of the ionospheric response during the storm. This comparison reveals various agreements between the simulation and observations, such as spatial patterns in the FAC timescales due to expansion/contraction of the polar cap. Finally, we investigate the correlation of the FAC with other simulated global parameters to determine which are most relevant to the overall stormtime response. These results help to reveal the importance of different aspects of the magnetosphere-ionosphere system in influencing the coupling timescales, and thus in determining the potential impacts of a severe event.