Magnetic Field Structures Triggering Solar Flares and Coronal Mass Ejections

Abstract

Solar flares and coronal mass ejections (CMEs), the most catastrophic eruptions in our solar system, have been known to affect terrestrial environments and infrastructure. However, because their triggering mechanism is still not sufficiently understood, our capacity to predict the occurrence of solar eruptions and to forecast space weather is substantially hindered. Even though various models have been proposed to determine the onset of solar eruptions, the types of magnetic structures capable of triggering these eruptions are still unclear. In this study, we solved this problem by systematically surveying the nonlinear dynamics caused by a wide variety of magnetic structures in terms of three-dimensional magnetohydrodynamic simulations. As a result, we determined that two different types of small magnetic structures favor the onset of solar eruptions. These structures, which should appear near the magnetic polarity inversion line (PIL), include magnetic fluxes reversed to the potential component or the nonpotential component of major field on the PIL. In addition, we analyzed two large flares, the X-class flare on December 13, 2006 and the M-class flare on February 13, 2011, using imaging data provided by the Hinode satellite, and we demonstrated that they conform to the simulation predictions. These results suggest that forecasting of solar eruptions is possible with sophisticated observation of a solar magnetic field, although the lead time must be limited by the time scale of changes in the small magnetic structures.

Triggering an Eruptive Flare by Emerging Flux in a Solar Active-Region Complex

“…This is true for emergence both near the PIL and near the edge of the arcade field overlying the PIL, although the latter case was found to be restricted in the range of distances to the PIL (Xu, Fang, and Chen, 2008). The reconnection destabilizes a pre-existing flux rope (Chen and Shibata, 2000;Xu, Chen, and Fang, 2005) or transforms the inner part of a pre-existing sheared arcade into an unstable flux rope (Kusano et al, 2012;Kaneko and Yokoyama, 2014). Alternatively, the emerging flux itself forms a flux rope, and the reconnection of this flux rope with a pre-existing arcade is essential for the removal of the rope's own stabilizing envelope, such that the rope will erupt (Archontis and Török, 2008;Archontis and Hood, 2012;Leake, Linton, and Antiochos, 2014).…”

“…In the case of the former, the eruption occurs when newly emerged magnetic fields appear in a region of pre-existing flux, often in or in close proximity to a filament channel (e.g., Rust, 1972;Martin et al, 1982;Demoulin et al, 1993;Feynman and Martin, 1995;Li et al, 2000;Sakajiri et al, 2004;Schrijver, 2009;Sun et al, 2012). This can lead to the formation of a magnetic flux rope through bodily emergence (Low, 1996;Lites, 2005), by reconnection within an emerged magnetic arcade (Manchester et al, 2004;Archontis and Hood, 2010), or by reconnection with the pre-existing flux (Kusano et al, 2012). A number of variants for the triggering of eruptions by emerging flux have been proposed (see Section 5 for details).…”

“…A number of variants for the triggering of eruptions by emerging flux have been proposed (see Section 5 for details). According to nearly all of them, the presence of pre-existing flux, orientated in a direction that facilitates reconnection with the emerging flux, is crucial to the eruption of the coronal flux rope (e.g., Chen and Shibata, 2000;Archontis and Török, 2008;Kusano et al, 2012;Leake, Linton, and Antiochos, 2014). Otherwise, a specific, narrow range of locations (Xu, Fang, and Chen, 2008) or a large amount of emerging flux (Kaneko and Yokoyama, 2014) is required.…”

Numerical Simulations of a Flux Rope Ejection

“…The thether-cutting process following the flux cancellation results are crucial to enhance the ejection and the MHD simulation roughly reproduces the EUV image from AIA of that flare. Kusano et al (2012) conducted numerical simulations, where two sheared arcades are in contact and the ejection is onset when the relative position is favourable to flux cancellation.…”

“…The numerical simulation developed in Roussev et al (2012) describes how the global reorganization of the solar corona that follows from the emergence of flux generates CME and how reconnection can enhance fast CMEs. Leake et al (2014) describe that a CME can develop from a flux emergence event only if the flux rope is allowed to reconnect with the external field, but the major acceleration of the flux rope occurs when also internal reconnection is onset, which plays the role of the flux cancellation. All these models give present plausible explanation of the mechanism that triggers the ejection of a flux rope.…”