Signatures of Nonideal Plasma Evolution During Substorms Obtained by Mining Multimission Magnetometer Data

Abstract

Substorm-type evolution of the Earth's magnetosphere is investigated by mining more than two decades of spaceborne magnetometer data from multiple missions including the first two years (2016)(2017) of the Magnetospheric MultiScale mission. This investigation reveals interesting features of plasma evolution distinct from ideal magnetohydrodynamics (MHD) behavior: X-lines, thin current sheets, and regions with the tailward gradient of the equatorial magnetic field B z . X-lines are found to form mainly beyond 20 R E , but for strong driving, with the solar wind electric field exceeding ∼ 5mV/m, they may come closer. For substorms with weaker driving, X-lines may be preceded by redistribution of the magnetic flux in the tailward B z gradient regions, similar to the magnetic flux release instability discovered earlier in PIC and MHD simulations as a precursor mechanism of the reconnection onset. Current sheets in the growth phase may be as thin as 0.2 R E , comparable to the thermal ions gyroradius, and at the same time, as long as 15 R E . Such an aspect ratio is inconsistent with the isotropic force balance for observed magnetic field configurations. These findings can help resolve kinetic mechanisms of substorm dipolarizations and adjust kinetic generalizations of global MHD models of the magnetosphere. They can also guide and complement microscale analysis of nonideal effects. Plain Language SummaryThe sun emits a steam of charged particles called the solar wind that flows past the Earth interacting with the planet's dipole magnetic field. This stretches the dipolar magnetic field away from the sun on the nightside of the planet storing energy in the stretched field. Once every few hours, this stretched configuration suddenly becomes more dipolar bringing particles and magnetic flux closer to the planet and powering aurora in the polar regions. During these processes, termed substorms, the gas of charged particles, protons, and electrons trapped by the dipole and known as plasma, behaves largely as a perfectly conducting fluid. However, only deviations from this ideal conducting plasma behavior can explain the substorm mechanisms. We mine two decades of spacecraft magnetometer data from multiple missions to form swarms of thousands of synthetic probes. They help reveal effects of nonideal plasma evolution during substorms, which cannot be captured by direct in situ observations because of their extreme paucity.However, more than half a century ago, while examining the magnetospheric convection cycle, Dungey (1961) concluded that there are regions with topological singularities of the magnetic field, where ideal MHD

Data Mining Reconstruction of Magnetotail Reconnection and Implications for Its First-Principle Modeling

“…It was found that such a data-mining (DM) method resolves the formation of embedded TCSs in the growth phase of substorms and their decay after the substorm onset. It also resolves the formation of the near-Earth X-lines during substorms [27].…”

“…Since the resulting magnetic field geometry is determined by the instantaneous KNN swarm of virtual probes, its time resolution is largely determined by the global parameter cadence. This is seen, for instance, from rapid substorm dipolarizations reproduced by the KNN method in [27] (Fig. 8i), when the B z field increases from 3 to 10 nT in 5 min over a significant part of the magnetotail.…”

“…Gray dots show the projections of the spacecraft coordinates on the equatorial plane, where the normal magnetic field component B z is color coded. (C) The resulting magnetic field lines (black) and the equatorial field B z (color-coded) assuming zero tilt angle (adapted from [27]).…”

Characteristics of Energetic Electrons Near Active Magnetotail Reconnection Sites: Tracers of a Complex Magnetic Topology and Evidence of Localized Acceleration

“…In the model results shown in Figure 3a, reconnection should have been occurring at and around the earthward edge of the Bz = 0 contour line, which we have highlighted with blue “X”s. A magnetic flux rope structure (see Sitnov et al., 2019) formed within the Bz = 0 contour of the reconstructed field. Furthermore, Figures 3b–3d show magnetic field lines projected onto three radial planes: one going through the MMS location (Figure 3c) and one at ±0.5 RE in azimuth on either side of MMS (Figures 3b and 3d).…”

Configuration of the Earth’s Magnetotail Current Sheet

“…Note observationally L < 10 ρ with ρ calculated through B lobe is typical of the thinnest magnetotail current sheets. Thin current sheets are generally embedded into much broader plasma sheet (Petrukovich et al., 2011; Runov et al., 2006; V. A. Sergeev et al., 1993; Sitnov, Stephens, et al., 2019), and magnetic field magnitude at the thin current sheet boundary is about ∼ B lobe /3 (Petrukovich et al., 2015). The ratio recalculated with magnetic field at the thin current sheet boundary (instead of B lobe ) would be a factor of ∼10 smaller, that is L / ρ ∼ 10 with B lobe (used through the study) corresponds to a current sheet thickness of about one ion Larmor radius with actual magnetic field magnitude ∼ B lobe /3 (see details in, e.g., Artemyev et al., 2010).…”