Eastward Transients in the Dayside Ionosphere II: A Parallel-plate Capacitor-Like Effect

TL;DR

This work documents fast, eastward electric-field transients in the dayside ionosphere near the cusp occurring on closed magnetic field-lines during geomagnetic storms. Using the high-resolution icebear coherent-scatter radar data alongside SuperDARN and DMSP cusp measurements, it shows eastward E-region motions up to $5×10^3$ m s⁻¹ that accompany chorus-wave–driven precipitation. The authors propose a parallel-plate capacitor-like mechanism in which polarization fields and turbulent Hall currents form between cusp-related charge regions, producing localized $E×B$-drift–like dynamics that are not captured by standard cusp convection models. This mechanism could be a common feature of storm-time dayside electrodynamics, with implications for ionospheric conductance, Joule heating, and the interpretation of global convection models.

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 matching temporal characteristics as that of the magnetospheric observations. We detailed this in a companion paper. 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 magnetospheric equator or by proton precipitation characteristics in the cusp, forming a `parallel-plate capacitor-like effect'. We propose that transient eastward electrodynamic bursts in the dayside ionosphere might be a common, albeit previously unresolved, feature of geomagnetic storms.

Figures (6)

• Figure 1: Panel a): Schematic drawing of the cusp, its extended mantle, and the motion of traditional poleward-moving aurora forms, as well as their expected seeding of polar cap patches. Panel b): Schematic drawing of the greater cusp-region, showing a mix of low-energy electrons (pink shaded area, minus signs) and protons (light blue shaded area, plus signs), with the most intense proton aurora on its equatorward edge. A region of high-energy diffuse aurora lies somewhere to the southeast of the cusp, and the two regions are separated by the open-closed field-line boundary. An strong equatorward electric field forms between the regions, driving turbulent electrojets, currents whose laminar form is broken up and developing the condition. The action of the turbulent Hall drifts (red arrow) may push islands of structured ionization into the polar cap, if aided by dayside reconnection.
• Figure 2: Solar wind parameters and geomagnetic activity index values for the period leading up to two major geomagnetic storms, occurring on 10 May 2024 (panels a, b) and 23 April 2023 (c, d). A shaded gray area denotes the duration of the two events under study. Panels a) and c) show the interplanetary magnetic field $B_Z$ (black) and $B_Y$ (red) components timeshifted to the bowshock papitashviliOMNIHourlyData2020. Panels b) and d) show the Sym-H geomagnetic storm index (black, left axis) and the solar wind dynamic pressure (red, right axis). Various features are annotated with arrows (SSC stands for sudden storm commencement).
• Figure 3: Observations made by the NOAA-18 spacecraft davis_history_2007 of precipitating electrons (a, d), ions (b, e), using the TED 0$^\circ$ telescope, as well as trapped electrons (c, f), using the MEPED 30$^\circ$ telescope evans_polar_2000, during two space-ground conjunctions; one occurring on 10 May 2024 (a--c) and one occurring on 23 April 2023 (d--f). Magnetic latitude (calculating using AACGM bakerNewMagneticCoordinate1989) is shown on the left $y$-axis, while time in UT is shown on the right $y$-axis. In panels a), b), d), and e), black and red lines indicate low- and high-energy particle fluxes respectively, while panels c) and f) show three distinct high-energy fluxes. In all panels, the latitudinal distribution of icebear echoes (with locations traced along Earth's magnetic field-lines) are shown with a green line. A grey shaded region indicates where Earth's magnetic field-lines are inferred to be closed.
• Figure 4: icebear cluster motions (green) compared to F-region velocities from Superdarn, both observations (red) and model-derived (blue). The leftmost column shows the temporal development in velocities while the rightmost column shows histograms; the four top panels show the 10 May 2024 event, while the bottom four panels show the 23 April 2023 event. Tracked icebear echo clusters were selected for containing a minimum of 300 echoes, a minimum duration of 6 seconds, as well as variability (68-percent confidence intervals of the linear fits of the echo cluster motion measurements) that did not exceed 2/3 of the cluster speed itself (variability is shown by green errorbars). For poleward motions (panels a, b, e, and f), Doppler shifts measured by the Saskatoon Superdarn radar are shown in red (using beam 4, which is pointing in the direction of the geomagnetic north pole).
• Figure 5: icebear echo velocities in geospace are shown as red arrows (speed indicated with a colorscale), and Superdarn global convection velocity are shown as blue arrows, for a 40 minute interval on 10 May 2024. See Figure \ref{['fig:velocities0']}a--d) for a detailed description of the directions and magnitudes of the measured and estimated velocities.