Timescales of Ionospheric Field-Aligned Currents during a Geomagnetic Storm: Global Magnetospheric Simulations

Geomagnetic storms generate a complex and highly time-dependent response in the magnetosphere-ionosphere system. Enhancement in field-aligned currents (FACs) can be very localised, and so accurately predicting the stormtime response of the ionosphere is crucial in forecasting the potential impacts of a severe space weather event at a given location on the Earth. Global MHD simulations provide a means to model ionospheric conditions in real-time for a given geomagnetic storm, allowing direct comparison to space- and ground-based observations from which the observations can be placed in global context to better understand the physical drivers behind the system's response.                   

Using the Gorgon MHD code and driving with upstream data from the ACE spacecraft, we simulate the state of the magnetosphere-ionosphere system during a geomagnetic storm commencing on 3^rd May 2014. To elucidate the characteristic timescales of the system response during this event, we adopt a novel approach originally applied by Shore et al. (2019) to ground magnetic field data from SuperMAG, and by Coxon et al. (2019) to FAC data from AMPERE. In this method the simulated FAC at each point on the ionospheric grid is cross-correlated with solar wind time-series for time lags of up to several hours, and the lag with the strongest correlation is identified.

From this we construct maps of the characteristic response timescale and strength of correlation in the ionosphere to IMF B_y and B_z, and interpret these results in terms of the varying stormtime FAC morphology by comparing the simulation results to observations by AMPERE and SuperMAG during this same event. Finally, we identify sources of asymmetry in the ionospheric response, such as that between day/night and north/south, relating these to asymmetries in magnetospheric dynamics such as magnetopause and magnetotail reconnection, and changes in global convection as the system reconfigures. This will reveal the importance of different aspects of magnetosphere-ionosphere system in influencing the coupling timescales, as well as the role of onset time in determining the potential impacts of a severe event.

References:

Shore, R. M., Freeman, M. P., Coxon, J. C., Thomas, E. G., Gjerloev, J. W., & Olsen, N. (2019). Spatial variation in the responses of the surface external and induced magnetic field to the solar wind. Journal of Geophysical Research: Space Physics, 124. https://doi.org/10.1029/2019JA026543

Coxon, J. C., Shore, R. M., Freeman, M. P., Fear, R. C., Browett, S. D., Smith, A. W., et al. (2019). Timescales of Birkeland currents driven by the IMF. Geophysical Research Letters, 46, 7893– 7901. https://doi.org/10.1029/2018GL081658