Auroral signatures of ballooning instability and plasmoid formation processes in the near-Earth magnetotail

TL;DR

The paper tests whether ballooning instability and consequent plasmoid formation in the near-Earth magnetotail can explain auroral beads and substorm onset. It combines a generalized Harris-sheet, resistive MHD framework from the NIMROD code with TREx-ATM auroral mapping to project tail-field-aligned currents to the ionosphere and compare against THEMIS ASI observations for a March 5, 2009 substorm. A key finding is that a single long-wavelength perturbation cannot reproduce the observed auroral evolution, whereas a double-mode initiation with a shorter-wavelength component yields azimuthally spaced beads, a poleward-expanding arc, and plasmoid formation consistent with observations and in-situ data, implying near-Earth neutral line formation around $10$–$12\,R_E$. The work supports ballooning instability as a plausible onset mechanism and highlights the need for two-fluid/FLR physics in future high-resolution magnetotail-ionosphere coupling models.

Abstract

The nonlinear development of ballooning instability and the subsequently induced plasmoid formation in the near-Earth magnetotail demonstrated in MHD simulations has been proposed as a potential trigger mechanism for substorm onset over the past decade, and their connections to the in-situ satellite and ground all-sky auroral optical observations have been a subject of continued research. In this work, a set of THEMIS substorm onset events with good conjunction of auroral observations has been selected for comparative simulation study, whose pre-onset magnetotail configuration and conditions are inferred from in-situ data and compared with the onset conditions of ballooning instability obtained in our MHD simulations. The evolution of the near-Earth magnetotail is followed, where the signatures of ballooning instability and the plasmoid formation are extracted from simulations and compared with the magnetic fields and flow patterns within the magnetotail region from observation data. The field-aligned current (FAC) density is evaluated at the Earth side boundary of the magnetotail domain of simulation, which is further mapped along magnetic field lines to the auroral ionosphere and compared with the auroral pattern and evolution there in terms of growth rate, dominant wavenumber, and absolute auroral intensities. Such validation efforts are also the first step towards the development of a self-consistent coupling model that includes the magnetotail-ionosphere interaction in the substorm onset process.

Figures (4)

• Figure 1: THEMIS-A (a: upper), THEMIS-D (b: lower left) and THEMIS-E (c: lower right) observations of magnetic field (1st row), ion velocity (2nd row), perpendicular ion velocity (3rd row), ion number density (4th row) and ion/magnetic pressure (5th row) as functions of time during the 2009, March 5 ("20090305") substorm event.
• Figure 2: (a: upper left) The ($X_{\rm GSM}$, $Y_{\rm GSM}$) coordinates of THEMIS satellite orbit trajectories; (b: upper right) the $B_z$ component of magnetic field measured by TH-A (dark solid line) and the fitted value of $B_z$ (blue dashed line), (c: lower left) the lobe magnetic field magnitude $B_{\rm lobe}$ measured by TH-D (red solid line) and TH-E (dark solid line) and the fitted value of $B_{\rm lobe}=40nT$ (blue dashed line), and (d: lower right) the perpendicular (dark solid line) and parallel (green solid line) electron temperature measured by TH-D and the fitted electron temperature (blue dashed line) as functions of time during the 2009, March 5 ("20090305") substorm event.
• Figure 3: ASI observation of auroral emission from 015630 UT to 015918 UT during the 20090305 substorm event .
• Figure 4: Equatorial $B_z(z=0)$ profile as a function of $x$ (upper) and magnetic field streamlines in the meridian plane ($y=0$) (lower).