Solar wind-magnetosphere-ionosphere coupling during the October 2024 storms

Two geomagnetic storms occurred in October 2024, driven by the impact of a series of interplanetary coronal mass ejections (ICMEs) on the magnetosphere.  The first was a moderate storm, with peak Sym-H near -150 nT, whereas the second was intense, Sym-H reaching -340 nT.  We compare and contrast the magnetospheric dynamics in each case, using observations of field-aligned currents (FACs) from the Active Magnetospheric and Planetary Electrodynamics Response Experiment (AMPERE) and ground magnetic perturbations observed by SuperMAG. The first storm responded linearly to solar wind driving, quantified by a dayside reconnection coupling function, and displayed typical substorm dynamics.  The response during the second storm suggests that the cross-polar cap potential (CPCP) saturated, and that the dynamics of the inner magnetosphere were complicated.  Magnetospheric compression by high solar wind pressure during the second storm produced elevated FAC magnitudes, indicating that both convection and compression control magnetosphere-ionosphere coupling.  We introduce a new FAC pattern complexity index which shows quantitively that the FAC pattern during the first storm largely retained the region 1 and 2 configuration associated with twin-cell ionospheric convection, but that during the second storm the pattern became more highly structured.  We conclude that storm intensity should not solely be quantified by Sym-H but also by other aspects of the magnetospheric response to solar wind disturbances.