The Mechanism for the Energy Buildup Driving Solar Eruptive Events

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractThe underlying origin of solar eruptive events (SEEs), ranging from giant coronal mass ejections to small coronalhole jets, is that the lowest-lying magnetic flux in the Sun's corona undergoes continual buildup of stress and free energy. This magnetic stress has long been observed as the phenomenon of "filament channels:" strongly sheared magnetic field localized around photospheric polarity inversion lines. However, the mechanism for the stress buildup-the formation of filament channels-is still debated. We present magnetohydrodynamic simulations of a coronal volume that is driven by transient, cellular boundary flows designed to model the processes by which the photosphere drives the corona. The key feature of our simulations is that they accurately preserve magnetic helicity, the topological quantity that is conserved even in the presence of ubiquitous magnetic reconnection. Although small-scale random stress is injected everywhere at the photosphere, driving stochastic reconnection throughout the corona, the net result of the magnetic evolution is a coherent shearing of the lowest-lying field lines. This highly counterintuitive result-magnetic stress builds up locally rather than spreading out to attain a minimum energy state-explains the formation of filament channels and is the fundamental mechanism underlying SEEs. Furthermore, this process is likely to be relevant to other astrophysical and laboratory plasmas.

Multiwavelength Study of Equatorial Coronal-hole Jets

“…However, the newly emerged bipoles do not erupt immediately, which indicates that they emerge with insufficient free energy to enable eruption. Therefore, we conclude that shearing and/or rotational photospheric motions are the most likely sources of the energy buildup that forms the filament channel and is released through eruption (e.g., Wyper et al 2018b), as in the helicity condensation model (Antiochos 2013;Knizhnik et al 2017;Dahlin et al 2018). Some of the larger analyzed events clearly exhibit rotational and/or shearing displacements between the minority polarity and its surroundings (e.g., see animations of Figs.…”

Magnetic Helicity Condensation and the Solar Cycle

“…High-resolution numerical simulations (Knizhnik et al 2015(Knizhnik et al , 2017aZhao et al 2015) have confirmed the accumulation of helicity at the perimeter of a compact region of such vortical flows in a plane-parallel corona, supporting the validity of the three-stage process described above. In a more definitive study of helicity condensation, in a bipolar magnetic region with a true PIL at the solar surface, Knizhnik et al (2017b) formed a filament channel having concentrated shear and dipped field lines, as observed on the Sun.…”

Determining the Transport of Magnetic Helicity and Free Energy in the Sun’s Atmosphere

“…Taylor's hypothesis has been largely verified in laboratory devices, such as reverse field pinches, where conducting walls with B · n| S = 0 isolate the plasma (Butt et al 1976). Furthermore, helicity conservation has been observed in numerous MHD simulations with high Lundquist number (e.g., MacNeice et al 2004;Knizhnik et al 2017). The magnetic helicity H is also a robust invariant in periodic geometries in the absence of a mean magnetic field.…”