M. G. Linton

Reconnection of Twisted Flux Tubes as a Function of Contact Angle

The collision and reconnection of magnetic flux tubes in the solar corona has been proposed as a mechanism for solar flares and in some cases as a model for coronal mass ejections. We study this process by simulating the collision of pairs of twisted flux tubes with a massively parallel, collocation, viscoresistive, magnetohydrodynamic code using up to 256 × 256 × 256 Fourier modes. Our aim is to investigate the energy release and possible global topological changes that can occur in flux-tube reconnection. We have performed a number of simulations for different angles between the colliding flux tubes and for either co- or counterhelicity flux tubes. We find the following four classes of interaction: (1) bounce (no appreciable reconnection), (2) merge, (3) slingshot (the most efficient reconnection), and (4) tunnel (a double reconnection). We will describe these four classes of flux-tube reconnection and discuss in what range of parameter space each class occurs and the implications our results have for models of flares and coronal mass ejections.

A Model for Patchy Reconnection in Three Dimensions

We show, theoretically and via MHD simulations, how a short burst of localized reconnection on a current sheet creates a pair of reconnected flux tubes. We focus on the post-reconnection evolution of these flux tubes, studying their velocities and shapes. We find that slow-mode shocks propagate along these reconnected flux tubes, releasing magnetic energy as in steady state Petschek reconnection. The geometry of these three-dimensional shocks, however, differs significantly from the classical two-dimensional geometry. They propagate along the flux tube legs in four isolated fronts, whereas in the two-dimensional Petschek model, they form a continuous, stationary pair of V-shaped fronts. We find that the cross sections of these reconnected flux tubes appear as teardrop-shaped bundles of flux propagating away from the reconnection site. Based on this, we argue that the descending coronal voids seen by Yohkoh SXT, LASCO, and TRACE are reconnected flux tubes descending from a flare site in the high corona, for example after a coronal mass ejection. In this model, these flux tubes would then settle into equilibrium in the low corona, forming an arcade of postflare coronal loops.

Simulations of Emerging Magnetic Flux. I. The Formation of Stable Coronal Flux Ropes

We present results from three-dimensional visco-resistive magnetohydrodynamic simulations of the emergence of a convection zone magnetic flux tube into a solar atmosphere containing a pre-existing dipole coronal field, which is orientated to minimize reconnection with the emerging field. We observe that the emergence process is capable of producing a coronal flux rope by the transfer of twist from the convection zone, as found in previous simulations. We find that this flux rope is stable, with no evidence of a fast rise, and that its ultimate height in the corona is determined by the strength of the pre-existing dipole field. We also find that although the electric currents in the initial convection zone flux tube are almost perfectly neutralized, the resultant coronal flux rope carries a significant net current. These results suggest that flux tube emergence is capable of creating non-current-neutralized stable flux ropes in the corona, tethered by overlying potential fields, a magnetic configuration that is believed to be the source of coronal mass ejections.

MULTI-FLUID SIMULATIONS OF CHROMOSPHERIC MAGNETIC RECONNECTION IN A WEAKLY IONIZED REACTING PLASMA

We present results from the first self-consistent multi-fluid simulations of chromospheric magnetic reconnection in a weakly ionized reacting plasma. We simulate two-dimensional magnetic reconnection in a Harris current sheet with a numerical model which includes ion-neutral scattering collisions, ionization, recombination, optically thin radiative loss, collisional heating, and thermal conduction. In the resulting tearing mode reconnection the neutral and ion fluids become decoupled upstream from the reconnection site, creating an excess of ions in the reconnection region and therefore an ionization imbalance. Ion recombination in the reconnection region, combined with Alfvenic outflows, quickly removes ions from the reconnection site, leading to a fast reconnection rate independent of Lundquist number. In addition to allowing fast reconnection, we find that these non-equilibria partial ionization effects lead to the onset of the nonlinear secondary tearing instability at lower values of the Lundquist number than has been found in fully ionized plasmas. These simulations provide evidence that magnetic reconnection in the chromosphere could be responsible for jet-like transient phenomena such as spicules and chromospheric jets.

Gas-dynamic Shock Heating of Post-flare Loops Due to Retraction Following Localized, Impulsive Reconnection

We present a novel model in which shortening of a magnetic flux tube following localized, three-dimensional reconnection generates strong gas-dynamic shocks around its apex. The shortening releases magnetic energy by progressing away from the reconnection site at the Alfv´ en speed. This launches inward flows along the field lines whose collision creates a pair of gas-dynamic shocks. The shocks raise both the mass density and temperature inside the newly shortened flux tube. Reconnecting field lines whose initial directions differ by more that 100 ‐ can produce a concentrated knot of plasma hotter that 20 MK, consistent with observations. In spite of these high temperatures, the shocks convert less than 10% of the liberated magnetic energy into heat — the rest remains as kinetic energy of bulk motion. These gas-dynamic shocks arise only when the reconnection is impulsive and localized in all three dimensions; they are distinct from the slow magnetosonic shocks of the Petschek steady-state reconnection model. Subject headings: MHD — shock waves — Sun: flares

Reconnection of a Kinking Flux Rope Triggering the Ejection of a Microwave and Hard X-Ray Source II. Numerical Modeling

Numerical simulations of the helical (m=1) kink instability of an arched, line-tied flux rope demonstrate that the helical deformation enforces reconnection between the legs of the rope if modes with two helical turns are dominant as a result of high initial twist in the range Φ≳6π. Such a reconnection is complex, involving also the ambient field. In addition to breaking up the original rope, it can form a new, low-lying, less twisted flux rope. The new flux rope is pushed downward by the reconnection outflow, which typically forces it to break as well by reconnecting with the ambient field. The top part of the original rope, largely rooted in the sources of the ambient flux after the break-up, can fully erupt or be halted at low heights, producing a “failed eruption.” The helical current sheet associated with the instability is squeezed between the approaching legs, temporarily forming a double current sheet. The leg – leg reconnection proceeds at a high rate, producing sufficiently strong electric fields that it would be able to accelerate particles. It may also form plasmoids, or plasmoid-like structures, which trap energetic particles and propagate out of the reconnection region up to the top of the erupting flux rope along the helical current sheet. The kinking of a highly twisted flux rope involving leg – leg reconnection can explain key features of an eruptive but partially occulted solar flare on 18 April 2001, which ejected a relatively compact hard X-ray and microwave source and was associated with a fast coronal mass ejection.

Simulations of Emerging Magnetic Flux. II. The Formation of Unstable Coronal Flux Ropes and the Initiation of Coronal Mass Ejections

We present results from three-dimensional magnetohydrodynamic simulations of the emergence of a twisted convection zone flux tube into a pre-existing coronal dipole field. As in previous simulations, following the partial emergence of the sub-surface flux into the corona, a combination of vortical motions and internal magnetic reconnection forms a coronal flux rope. Then, in the simulations presented here, external reconnection between the emerging field and the pre-existing dipole coronal field allows further expansion of the coronal flux rope into the corona. After sufficient expansion, internal reconnection occurs beneath the coronal flux rope axis, and the flux rope erupts up to the top boundary of the simulation domain (~36?Mm above the surface). We find that the presence of a pre-existing field, orientated in a direction to facilitate reconnection with the emerging field, is vital to the fast rise of the coronal flux rope. The simulations shown in this paper are able to self-consistently create many of the surface and coronal signatures used by coronal mass ejection (CME) models. These signatures include surface shearing and rotational motions, quadrupolar geometry above the surface, central sheared arcades reconnecting with oppositely orientated overlying dipole fields, the formation of coronal flux ropes underlying potential coronal field, and internal reconnection which resembles the classical flare reconnection scenario. This suggests that proposed mechanisms for the initiation of a CME, such as magnetic breakout, are operating during the emergence of new active regions.

DISTRIBUTION OF ELECTRIC CURRENTS IN SOLAR ACTIVE REGIONS

There has been a long-standing debate on the question of whether or not electric currents in solar active regions are neutralized. That is, whether or not the main (or direct) coronal currents connecting the active region polarities are surrounded by shielding (or return) currents of equal total value and opposite direction. Both theory and observations are not yet fully conclusive regarding this question, and numerical simulations have, surprisingly, barely been used to address it. Here we quantify the evolution of electric currents during the formation of a bipolar active region by considering a three-dimensional magnetohydrodynamic simulation of the emergence of a sub-photospheric, current-neutralized magnetic flux rope into the solar atmosphere. We find that a strong deviation from current neutralization develops simultaneously with the onset of significant flux emergence into the corona, accompanied by the development of substantial magnetic shear along the active regions polarity inversion line. After the region has formed and flux emergence has ceased, the strong magnetic fields in the regions center are connected solely by direct currents, and the total direct current is several times larger than the total return current. These results suggest that active regions, the main sources of coronal mass ejections and flares, are born with substantial net currents, in agreement with recent observations. Furthermore, they support eruption models that employ pre-eruption magnetic fields containing such currents.

Magnetic reconnection in a weakly ionized plasma

Magnetic reconnection in partially ionized plasmas is a ubiquitous phenomenon spanning the range from laboratory to intergalactic scales, yet it remains poorly understood and relatively little studied. Here, we present results from a self-consistent multi-fluid simulation of magnetic reconnection in a weakly ionized reacting plasma with a particular focus on the parameter regime of the solar chromosphere. The numerical model includes collisional transport, interaction and reactions between the species, and optically thin radiative losses. This model improves upon our previous work in Leake et al. [“Multi-fluid simulations of chromospheric magnetic reconnection in a weakly ionized reacting plasma,” Astrophys. J. 760, 109 (2012)] by considering realistic chromospheric transport coefficients, and by solving a generalized Ohms law that accounts for finite ion-inertia and electron-neutral drag. We find that during the two dimensional reconnection of a Harris current sheet with an initial width larger than the...

Three-dimensional Reconnection of Untwisted Magnetic Flux Tubes

Understanding the reconnection of magnetic fields in flux tubes is of key importance for modeling solar activity and space weather. We are therefore studying this process via three-dimensional MHD simulations. We report here on a simulation of the collision of a pair of perpendicular, untwisted magnetic flux tubes. We find that the collision proceeds in four stages. First, on contact, the tubes flatten out into wide sheets. Second, they begin to reconnect and the tearing mode instability is excited in the reconnection region. Third, the nonlinear evolution of the tearing mode creates a pair of reconnected flux tubes. Finally, these flux tubes reconnect with each other to coalesce into a single flux tube. We then report on a pair of simulations exploring how this behavior changes when the speed of flux tube collision is increased and when the magnetic resistivity is increased. Subject headings: MHD — Sun: flares — Sun: magnetic fields On-line material: mpeg animations