Transient, Turbulent Hall Currents in the Sunlit Terrestrial Ionosphere

The `cusp' emerges from a direct coupling between Earth's ionosphere and the solar wind, and the dynamic auroras found in the cusp-region have long been taken to reflect the time-history of pulsed magnetic reconnection between Earth's magnetic field and that of the Sun. However, the prevailing explanation is challenged by observations of chorus wave-driven energetic particle precipitation on closed magnetic field lines, which excited strong modulations to the ionospheric electric field. Intense Hall current channels formed along the open-closed field-line boundary, triggering the Farley-Buneman instability and the consequent proliferation of intense meter-scale turbulent waves in the bottomside ionosphere. The finding is unearthed using data mining techniques applied to a large database of coherent scatter radar echoes from small-scale ionospheric plasma turbulence. The emerging radar tracking technique allows for the ionospheric electric field to be monitored over time. Further evidence in favour of our explanation is found with ground-based observations of magnetospheric plasma convection and GPS scintillations, as well as in situ observations from a weather satellite and an inner-magnetosphere spacecraft. Our results suggest that during geomagnetic storms the electrodynamics of the cusp-region can at times be driven by wave-particle interactions in the dayside magnetosphere instead of the solar wind.