Geoelectric Field Caused by Flux Transfer Events in an Ionosphere-Coupled Vlasiator Simulation

TL;DR

The paper investigates how flux transfer events (FTEs) at Earth's magnetopause induce geoelectric fields at Earth's surface by employing an ionosphere-coupled Vlasiator simulation that resolves both the magnetosphere and ionosphere. It identifies FTEs as multi-flux-rope structures separated by 3D magnetic nulls, whose helicity is organized by the magnetopause $B_y$ in a purely southward IMF scenario, and demonstrates that pulsed, Alfvénic field-aligned currents propagate Earthward and map down to the ionosphere, generating rotating geoelectric-field patterns around the noon meridian. Through Biot–Savart-based field calculations and a Cagniard-type integral for the geoelectric field, the study links transient FACs to localized, high-magnitude geoelectric fields ($\sim$0.1–0.2 V/km) near the high-latitude footpoints of FTEs, with strongest fields near $\sim80^ ext{o}$ latitude and $\mathrm{MLT}\approx12$. The results reveal a causal chain from magnetopause reconnection to ground-level geoelectric signatures and traveling-convection-velocity patterns, offering baseline predictions for geomagnetically induced currents (GICs) and guiding future observational and modeling efforts under more realistic solar-wind and ground-conductivity conditions.

Abstract

We report on the relationship between flux transfer events (FTEs) at Earth's magnetopause and the geoelectric field that is induced near the FTEs' magnetic footpoints. We study this system using the global hybrid-Vlasov code Vlasiator, which has recently been extended to model ionospheric physics. We also highlight the significance of 3D magnetic null points, which in our simulation can separate the FTEs into multiple flux ropes. Near the null points, the coiled FTE magnetic field lines are rerouted towards Earth, so that the magnetic footpoints are planted near the Region 1 ionospheric current system. The helicities of the flux ropes are organized by the y-component (GSE) of the magnetic field at the Earth's magnetopause. This occurs in our simulation due to the absence of a y-component of the interplanetary magnetic field, which would normally supply the FTE guide field that determines the helicity. We observe Alfvenic, Earthward-flowing field-aligned currents generated near the magnetopause that correlate with the passage of FTEs nearby. These pulses of current coincide with the formation of rotational geoelectric field structures, that appear near the noon meridian and propagate around the auroral oval towards the nightside.

Figures (7)

• Figure 1: The FTEs at simulation time $t=1165\ \mathrm{s}$. The cutaway shows the $y<0$ half of the magnetopause, i.e. the $\beta^\star$=0.5 isosurface xu16horaites23. Yellow magnetospheric field lines are shown on the meridional plane for reference. Vlasiator's ionosphere is shown by a central sphere, and the region $r_B<r<r_m$ between the magnetospheric and ionospheric domains is shown in black. The parallel current $J_\parallel$ evaluated at the FTE O-lines is shown in a blue-red scale. Two Flux Ropes, 1 and 2, in the same FTE are traced from starting positions $\mathbf{x_1}$, $\mathbf{x_2}$ on either side of a 3D magnetic null point (a blue-red junction). Both magnetic field lines are open-closed, with one end at the ionospheric footpoints of cusp field lines.
• Figure 2: Ground electromagnetic fields near the footpoints of cusp fieldlines in the southern hemisphere. The northern and eastern geoelectric field components $E_\lambda$, $E_\phi$, showing also their respective correlations with $-dB_\phi/dt$, $dB_\lambda/dt$.
• Figure 3: The $\beta^\star=0.5$ magnetopause is shown head-on, at $t=1165\ \mathrm{s}$ in a spatial box $x\in [0, 14~ \mathrm{R_E}]$, $y\in [-14~\mathrm{R_E}, 14~ \mathrm{R_E}]$, $z\in [-14~\mathrm{R_E}, 14~ \mathrm{R_E}]$. The magnetopause surface is colored in a red-blue scale according to the magnetic field component $B_y$. Simulation cells containing null lines are colored by the parallel current $J_\parallel$ (green-purple), and the cell edges are highlighted in white (for O-lines) or black (for X-lines). The 3D null points, which we observe occur on the null lines at locations where $J_\parallel$ changes sign, are shown as spheres. The null points are colored based on their polarity: yellow if $\Pi=-1$ and black if $\Pi=1$. This figure is animated in movie S3.
• Figure 4: Schematic diagram showing the magnetic field of the FTE flux ropes, as projected in the simulation's $y$-$z$ plane. The current helicity (RH or LH) and $\text{sign}(J_\parallel)$ of the flux ropes are correlated with $\text{sign}(B_y)$ of the underlying dayside magnetosphere. These quantities are thus organized into the four quadrants of the $y$-$z$ plane. The FTE current $\mathbf{J}$ flows in the $+\mathbf{\hat{y}}$ direction, in agreement with the magnetopause current. FACs flow to Earth's ionosphere, to a region containing the footpoints of the dayside cusp field lines.
• Figure 5: a) Proton plasma pressure $P_p$ in the plane $y=3.3~\mathrm{R_E}$, at time $t_0=1250$ s. The FTEs appear as regions of enhanced plasma pressure at the magnetopause, encircled by magnetic field lines (black). The projection of the curve $C_{345}$ in the plane is shown as a green line, with the points $\mathbf{x_3}$, $\mathbf{x_4}$, $\mathbf{x_5}$ shown as green dots. The coupling radius $r_C$ is shown as a grey dashed circle. b) The scaled parallel current $J_\parallel/B$, shown in a time elongation map where the x-axis represents the distance along the curve $C_{345}$ and the y-axis represents time. Each occasion that an FTE O-line crosses the curve $C_{345}$ is marked with an "O". The green dashed line, shown for reference, represents the space-time propagation of a field-aligned Alfvén wave, moving along $C_{345}$ from $\mathbf{x_4}$ to $\mathbf{x_3}$ and starting at initial time $t_0$. A blue star marks the arrival of an FAC pulse at the inner boundary at $t\approx1100\ \mathrm{s}$.