Coronal mass ejections (CMEs) and eruptive solar flares are the largest magnetic explosions in the solar system, yet the physical mechanism that initiates them and drives their explosive acceleration remains unsettled. A 2.5D axisymmetric (two spatial coordinates with all three magnetic-field components retained) magnetohydrodynamic simulation of the magnetic breakout model (Karpen, Antiochos & DeVore 2012) concluded that the explosive acceleration of a CME is a resistive instability: once the flare current sheet below the rising flux rope thins and tears into plasmoids, reconnection feeds back positively on the eruption, and the acceleration begins before and without any ideal-MHD instability such as the torus or kink instability. The unresolved problem is to determine whether the onset and explosive acceleration of solar eruptions is indeed driven by such resistive (reconnection) dynamics or by ideal MHD instability, and to establish the observable timing, particle-acceleration, and flare signatures that discriminate between the two.

Background

A coronal mass ejection (CME) is the eruption of 101410^{14}–101610^{16} g of magnetized plasma from the Sun's corona into interplanetary space, usually accompanied by an eruptive flare; the two phenomena are dynamically coupled through a vertical current sheet (a thin layer of intense electric current and magnetic-field reversal) in which magnetic reconnection — the change of magnetic-field topology with conversion of magnetic energy to kinetic and thermal energy — proceeds below the rising ejecta. The pre-eruption corona stores free magnetic energy in a twisted, sheared arcade of field lines anchored in the photosphere ('line-tied'), held down by overlying, less-sheared 'strapping' field.

Two broad classes of onset mechanism compete. Ideal-MHD mechanisms are non-dissipative: the eruption begins because the equilibrium or stability of the coronal field is lost. The torus instability (expansion of an arcade or flux rope when the overlying field decreases with height faster than a critical decay index; Kliem & Török 2006) and the helical kink instability (self-writhing of a twisted flux tube once its twist exceeds a threshold; Török & Kliem 2005) are the standard examples. Resistive mechanisms require reconnection to start or drive the eruption. In the magnetic breakout model (Antiochos, DeVore & Klimchuk 1999; Antiochos 1998), the multipolar topology of the source region allows reconnection at a current sheet above the sheared core ('breakout' reconnection) to remove the strapping field, releasing the core; in tether-cutting and plasmoid-driven pictures, reconnection below or within the erupting structure cuts the field lines that hold it down. Karpen, Antiochos & DeVore (2012, ApJ 760, 81) carried out a 2.5D axisymmetric simulation (two spatial coordinates with all three magnetic-field components retained) with adaptive mesh refinement and a scale-dependent anomalous resistivity and found that slow pre-eruption rise is governed by breakout reconnection, while the explosive acceleration begins only when the flare current sheet thins, becomes tearing-unstable, and fragments into magnetic islands (plasmoids) whose merging builds the erupting flux rope: a resistive (reconnection-driven) instability rather than an ideal one.

Several concepts are needed to weigh that conclusion. The tearing instability fragments a current sheet whose length-to-width aspect ratio is large; its nonlinear development, the plasmoid instability, produces chains of plasmoids and fast reconnection, with onset scalings in evolving sheets derived by Huang, Comisso & Bhattacharjee (2017) and Comisso et al. (2017). The Lundquist number SS (ratio of advective to diffusive magnetic transport) of coronal sheets is so large that Sweet–Parker-type sheets (the long, thin diffusion layers predicted by classical reconnection theory) cannot persist; realistic reconnection is plasmoid-mediated and possibly kinetic at the smallest scales, whereas the 2012 simulation imposed an effective resistivity tied to the computational grid. Observationally, the discriminant between ideal and resistive onset is chiefly temporal: whether the flare impulsive phase (rapid reconnection) precedes, coincides with, or lags the main acceleration of the CME, measured event by event from precise height–time and light-curve data (e.g., extreme-ultraviolet imaging by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) satellite and soft X-ray flux from the Geostationary Operational Environmental Satellites (GOES)), together with the decay index of the strapping field at the eruption onset height.

Problem Statement

Determine the physical mechanism of the onset and explosive acceleration of solar coronal mass ejections and eruptive flares — in particular, establish whether the explosive acceleration is a resistive instability, driven by tearing and plasmoid-mediated reconnection in the flare and breakout current sheets with the acceleration beginning before and independently of any ideal-MHD instability, or an ideal MHD instability such as the torus or kink instability, and whether one mechanism, or an event-dependent combination, governs real solar eruptions — and establish the timing and flare/particle-acceleration observables that discriminate between the mechanisms. The source paper (Karpen, Antiochos & DeVore 2012) states that its resistive-instability conclusion "requires verification", that agreement between the scaled simulation results and actual CME/flare parameters "should not be interpreted as independent verification of the model", and that the test is precise measurement of the relative timing between impulsive flare onset and CME acceleration in well-observed solar events. It further acknowledges as open the replacement of its numerically imposed resistivity in global eruption simulations by physically grounded (plasmoid-mediated and/or kinetic-scale) microphysics, the unexplained delay and initial predominance of downward-moving islands before the upward-moving, flux-rope-building plasmoids that trigger the explosive phase in its own 2.5D simulation, and the absence of particle-acceleration and high-energy radiation predictions for the magnetic nulls (points where the magnetic field vanishes) and current-sheet islands — the 'null/island dynamics'. A defensible determination of the mechanism of the onset and explosive acceleration — resistive reconnection, ideal-MHD instability, or an event-dependent combination governing real eruptions — from existing theory and observations, together with the flare/CME-timing, decay-index, and particle-acceleration observables that discriminate among the mechanisms, resolves the task; the source-acknowledged open items listed above are optional research directions, not additional required sub-goals.

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