Monday

Geomagnetic storms generate a complex and highly time-dependent response in the magnetosphere-ionosphere system. The loading of magnetic flux in the nightside results in reconnection at the magnetotail current sheet, generating ionospheric signatures. Asymmetries in the current sheet thus manifest in ionospheric field-aligned currents (FACs), and so understanding the timescales over which the current sheet reconfigures during strong, variable driving is crucial to describing the evolution of the FACs. For example, the current sheet twists in response to IMF By, and is hinged by the dipole tilt which changes diurnally and seasonally.

Global MHD simulations provide a powerful means to investigate magnetotail dynamics. To this end, we use the Gorgon MHD code to simulate a storm which commenced on 3rd May 2014. The initial 24 hours of this event were host to several key IMF features, including a switch in Bz from northward to southward and vice-versa, sharp reversals in By, and prolonged periods of southward IMF driving. This presents an ideal case study for the response timescales of the magnetotail and associated FACs.

We find that at 20 Earth radii downtail the current sheet twisting is most strongly correlated to IMF By at timescales of ~1 hour during southward IMF. The current sheet has a flatter, more idealised profile when the driving is stronger, responding incoherently between the inner and outer magnetotail when the IMF rapidly varies. The effect of a switch to northward IMF is well-established after ~3 hours, suggesting a bimodal behaviour, after which the twisting and hinging is more exaggerated. These effects become stronger further downtail, where the current sheet is more disturbed but still evolves over timescales consistent with those of global convection. We relate these results to simulated ionospheric conditions and observations from AMPERE during the same event, discussing the implications for solar wind-magnetosphere-ionosphere coupling.