Eastward transients in the dayside ionosphere. II. A parallel-plate capacitorlike effect.

Abstract

During the 23 April 2023 geospace storm, we observed chorus-wave-driven, energetic particle precipitation on closed magnetic field lines in the dayside magnetosphere. Simultaneously and in the ionosphere's bottom side, we observed signatures of impact ionization and strong enhancements in the ionospheric electric field, via radar detection of meter-scale turbulence, and with temporal characteristics matching those of the magnetospheric observations. We detail this in another work [M. F. Ivarsen et al., Phys. Rev. E 112, 045204 (2025)0031-900710.1103/r6bv-pzlq]. In the present article we place those observations into context with the dayside ionosphere and describe a remarkably similar event that took place during the May 2024 geospace superstorm. In both cases, fast, eastward-moving electric field structures were excited equatorward of the ionospheric cusp, on closed magnetic field lines, observations that challenge existing modes of explanation for electrodynamics in the cusp region, where most such observations are interpreted in the context of poleward-moving auroral forms. Instead, primarily eastward-moving electric field structures were associated with turbulent Hall currents that are perhaps characteristically excited during geospace storms by wave-particle interactions near the magnetospheric equator or by proton precipitation characteristics in the cusp, forming a parallel-plate capacitorlike effect. We propose that transient eastward electrodynamic bursts in the dayside ionosphere might be a common, albeit previously unresolved, feature of geomagnetic storms.