Eruption of a Buoyantly Emerging Magnetic Flux Rope

Abstract

We present a three-dimensional numerical magnetohydrodynamic simulation designed to model the emergence of a magnetic flux rope passing from below the photosphere into the corona. For the initial state, we prescribe a plane-parallel atmosphere that comprises a polytropic convection zone, photosphere, transition region, and corona. Embedded in this system is an isolated horizontal magnetic flux rope located 10 photospheric pressure scale heights below the photosphere. The flux rope is uniformly twisted, with the plasma temperature inside the rope reduced to compensate for the magnetic pressure. Density is reduced in the middle of the rope, so that this section buoyantly rises. The early evolution proceeds with the middle of the rope rising to the photosphere and expanding into the corona. Just as it seems the system might approach equilibrium, the upper part of the flux rope begins to separate from the lower, mass-laden part. The separation occurs through stretching of the field, which forms a current sheet, where reconnection severs the field lines to form a new system of closed flux. This flux then erupts into the corona. Essential to the eruption process are shearing motions driven by the Lorentz force, which naturally occur as the rope expands in the pressure-stratified atmosphere. The shearing motions transport axial flux and energy to the expanding portion of the magnetic field, driving the eruption. Subject headingg s: MHD -Sun: magnetic fields -Sun: photosphere

3D MHD Flux Emergence Experiments: Idealised Models and Coronal Interactions

“…In order for the magnetic field to remain coherent during this rise, the flux tube must be twisted by a sufficient amount (Moreno-Insertis and Emonet, 1996;Emonet and Moreno-Insertis, 1998). Hence, many researchers (Fan, 2001;Magara and Longcope, 2003;Manchester et al, 2004;Archontis et al, 2004;Murray et al, 2006;MacTaggart and Hood, 2010) have placed a horizontal, cylindrical, twisted flux tube near the top of the convection zone.…”

“…The rapid expansion of the magnetic field creates a region of reduced pressure (as indicated in Figure 4) and this acts as a sink for draining plasma. Apart from this cavitation, the expanding magnetic field produces another dynamically important effect -shearing along the polarity inversion line (PIL) between the two main photospheric polarities (Manchester, 2001;Fan, 2001;Manchester et al, 2004;Hood et al, 2009;MacTaggart and Hood, 2009). This shearing can be understood by examining the Lorentz force in the direction (y in this case) of the PIL, namely − ∂ ∂y…”

“…While this work was mainly 2D, it laid the foundations for the subsequent 3D simulations. Matsumoto et al (1998) were the first to investigate the emergence of a 3D flux tube from the the solar interior. They investigated how a cylindrical flux tube, unstable to a kink instability (Hood and Priest, 1979), can rise from the interior into the solar atmposhere.…”

Magnetic reconnection and energy release on the Sun and solar-like stars

“…Flux rope emergence has many caveats, e.g. in the concave-up parts under the flux rope axis plasma accumulates, leading to a fragmentation of the emerging flux rope (e.g., Magara 2004;Manchester et al 2004) which can only emerge through many smallscale reconnections (Pariat et al 2004). Nevertheless, characteristic magnetic patterns in emerging flux regions originating from the changing azimuthal component of a flux rope while crossing the photosphere, the so-called "magnetic tongues" (López-Fuentes et al 2000;Démoulin & Pariat 2008) indicate that there is an overall organization in the emerging flux tube, which is compatible with a global twist.…”

The Nonpotentiality of Coronae of Solar Active Regions, the Dynamics of the Surface Magnetic Field, and the Potential for Large Flares

“…Magnetohydrodynamic (MHD) simulations provide insight into the behavior of twisted flux ropes that emerge from within the convective envelope and currents that are induced by such emergence and by lateral displacements and twisting motions of photospheric field. Manchester et al (2004) and Fang et al (2012), for example, use MHD simulations to show how flux-rope emergence can build up magnetic shear in the corona through shearing flows at the polarity inversion line (PIL) and spot rotation. MHD studies by Török & Kliem (2003), Török et al (2014), and Dalmasse et al (2015) concur in that non-neutralized, i.e., net, electrical currents only occur if the twisting and shearing motions of the flux emergence that they prescribe build up magnetic shear at the PIL.…”