The Torus instability
TL;DR: In this paper, the expansion instability of a toroidal current ring in low-beta magnetized plasma is investigated, and the results are verified with experiments on spheromak expansion and with essential properties of solar coronal mass ejections.
Abstract: The expansion instability of a toroidal current ring in low-beta magnetized plasma is investigated. Qualitative agreement is obtained with experiments on spheromak expansion and with essential properties of solar coronal mass ejections, unifying the two apparently disparate classes of fast and slow coronal mass ejections.
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TL;DR: In this paper, a review on each stage of the CME phenomenon is presented, including their pre-eruption structure, their triggering mechanisms and the precursors indicating the initiation process, their acceleration and propagation.
Abstract: Coronal mass ejections (CMEs) are the largest-scale eruptive phenomenon in the solar system, expanding from active region-sized nonpotential magnetic structure to a much larger size. The bulk of plasma with a mass of ∼ 1011,1013 kg is hauled up all the way out to the interplanetary space with a typical velocity of several hundred or even more than 1000 km s−1, with a chance to impact our Earth, resulting in hazardous space weather conditions. They involve many other much smaller-sized solar eruptive phenomena, such as X-ray sigmoids, filament/prominence eruptions, solar flares, plasma heating and radiation, particle acceleration, EIT waves, EUV dimmings, Moreton waves, solar radio bursts, and so on. It is believed that, by shedding the accumulating magnetic energy and helicity, they complete the last link in the chain of the cycling of the solar magnetic field. In this review, I try to explicate our understanding on each stage of the fantastic phenomenon, including their pre-eruption structure, their triggering mechanisms and the precursors indicating the initiation process, their acceleration and propagation. Particular attention is paid to clarify some hot debates, e.g., whether magnetic reconnection is necessary for the eruption, whether there are two types of CMEs, how the CME frontal loop is formed, and whether halo CMEs are special.
679 citations
TL;DR: The current understanding of solar flares, mainly focused on magnetohydrodynamic (MHD) processes responsible for producing a flare, can be found in this article, where the authors present a review of the models proposed to explain the physical mechanism of flares, giving an comprehensive explanation of the key processes.
Abstract: This paper outlines the current understanding of solar flares, mainly focused on magnetohydrodynamic (MHD) processes responsible for producing a flare. Observations show that flares are one of the most explosive phenomena in the atmosphere of the Sun, releasing a huge amount of energy up to about 1032 erg on the timescale of hours. Flares involve the heating of plasma, mass ejection, and particle acceleration that generates high-energy particles. The key physical processes for producing a flare are: the emergence of magnetic field from the solar interior to the solar atmosphere (flux emergence), local enhancement of electric current in the corona (formation of a current sheet), and rapid dissipation of electric current (magnetic reconnection) that causes shock heating, mass ejection, and particle acceleration. The evolution toward the onset of a flare is rather quasi-static when free energy is accumulated in the form of coronal electric current (field-aligned current, more precisely), while the dissipation of coronal current proceeds rapidly, producing various dynamic events that affect lower atmospheres such as the chromosphere and photosphere. Flares manifest such rapid dissipation of coronal current, and their theoretical modeling has been developed in accordance with observations, in which numerical simulations proved to be a strong tool reproducing the time-dependent, nonlinear evolution of a flare. We review the models proposed to explain the physical mechanism of flares, giving an comprehensive explanation of the key processes mentioned above. We start with basic properties of flares, then go into the details of energy build-up, release and transport in flares where magnetic reconnection works as the central engine to produce a flare.
677 citations
TL;DR: In this article, a zero-β magnetohydrodynamic (MHD) simulation of an initially potential, asymmetric bipolar field, which evolves by means of simultaneous slow magnetic field diffusion and sub-Alfvenic, line-tied shearing motions in the photosphere, is used to analyze the physical mechanisms that form a three-dimensional coronal flux rope and later cause its eruption.
Abstract: We analyze the physical mechanisms that form a three-dimensional coronal flux rope and later cause its eruption. This is achieved by a zero-β magnetohydrodynamic (MHD) simulation of an initially potential, asymmetric bipolar field, which evolves by means of simultaneous slow magnetic field diffusion and sub-Alfvenic, line-tied shearing motions in the photosphere. As in similar models, flux-cancellation-driven photospheric reconnection in a bald-patch (BP) separatrix transforms the sheared arcades into a slowly rising and stable flux rope. A bifurcation from a BP to a quasi-separatrix layer (QSL) topology occurs later on in the evolution, while the flux rope keeps growing and slowly rising, now due to shear-driven coronal slip-running reconnection, which is of tether-cutting type and takes place in the QSL. As the flux rope reaches the altitude at which the decay index –∂ln B/∂ln z of the potential field exceeds ~3/2, it rapidly accelerates upward, while the overlying arcade eventually develops an inverse tear-drop shape, as observed in coronal mass ejections (CMEs). This transition to eruption is in accordance with the onset criterion of the torus instability. Thus, we find that photospheric flux-cancellation and tether-cutting coronal reconnection do not trigger CMEs in bipolar magnetic fields, but are key pre-eruptive mechanisms for flux ropes to build up and to rise to the critical height above the photosphere at which the torus instability causes the eruption. In order to interpret recent Hinode X-Ray Telescope observations of an erupting sigmoid, we produce simplified synthetic soft X-ray images from the distribution of the electric currents in the simulation. We find that a bright sigmoidal envelope is formed by pairs of -shaped field lines in the pre-eruptive stage. These field lines form through the BP reconnection and merge later on into -shaped loops through the tether-cutting reconnection. During the eruption, the central part of the sigmoid brightens due to the formation of a vertical current layer in the wake of the erupting flux rope. Slip-running reconnection in this layer yields the formation of flare loops. A rapid decrease of currents due to field line expansion, together with the increase of narrow currents in the reconnecting QSL, yields the sigmoid hooks to thin in the early stages of the eruption. Finally, a slightly rotating erupting loop-like feature (ELLF) detaches from the center of the sigmoid. Most of this ELLF is not associated with the erupting flux rope, but with a current shell that develops within expanding field lines above the rope. Only the short, curved end of the ELLF corresponds to a part of the flux rope. We argue that the features found in the simulation are generic for the formation and eruption of soft X-ray sigmoids.
618 citations
TL;DR: In this paper, the authors modeled the loss of confinement and eruption of a flux rope emerging quasi-statically into a preexisting coronal arcade field, and investigated two distinct mechanisms that led to the eruption of the flux rope.
Abstract: Using MHD numerical simulations in a three-dimensional spherical geometry, we model the loss of confinement and eruption of a flux rope emerging quasi-statically into a preexisting coronal arcade field. Our numerical experiments investigated two distinct mechanisms that led to the eruption of the flux rope. In one case, the overlying arcade field declines with height slowly such that the emerging flux rope remains confined until its self-relative magnetic helicity normalized by the square of the rope's flux reaches -1.4 and the flux rope becomes significantly kinked. The kinking motion causes rotation of the tube to an orientation that makes it easier for it to rupture through the arcade field, leading to an eruption. In the second case, the overlying field declines more rapidly with height, and the emerging flux rope is found to lose equilibrium and erupt via the torus instability when its self-relative magnetic helicity normalized by the square of its flux is only approximately -0.63, before it becomes kinked. The values of the total relative magnetic helicity of the entire coronal magnetic field (including both the flux rope and the arcade field) normalized by the square of the total magnetic flux are, on the other hand, of similar magnitudes for the two cases when the eruption takes place. We compare and contrast the eruptive properties and the posteruption states resulting from the two cases.
350 citations
TL;DR: The observations suggest that the instability of the magnetic flux rope triggers the eruption, thus making a major addition to the traditional magnetic-reconnection paradigm.
Abstract: Explosive energy releases in plasmas, such as in solar eruptions like flares and coronal mass ejections, are thought to be caused by magnetic reconnection in thin current sheets. Zhang et al. observed a magnetic flux rope during a solar eruption, highlighting its role in driving explosive energy releases.
349 citations