Inferring the Magnetohydrodynamic Structure of Solar Flare Supra-Arcade Plasmas From a Data-Assimilated Field Transport Model

Abstract

Supra-arcade fans are highly dynamic structures that form in the region above post-reconnection flare arcades. In these features the plasma density and temperature evolve on the scale of a few seconds, despite the much slower dynamics of the underlying arcade. Further, the motion of supra-arcade plasma plumes appears to be inconsistent with the low-beta conditions that are often assumed to exist in the solar corona. In order to understand the nature of these highly debated structures,it is, therefore, important to investigate the interplay of the magnetic field with the plasma. Here we present a technique for inferring the underlying magnetohydrodynamic processes that might lead to the types of motions seen in supra-arcade structures. Taking as a case study the 2011 October 22 event, we begin with extreme-ultraviolet observations and develop a time-dependent velocity field that is consistent with both continuity and local correlation tracking. We then assimilate this velocity field into a simplified magnetohydrodynamic simulation, which deals simultaneously with regions of high and low signal-to-noise ratio, thereby allowing the magnetic field to evolve self-consistently with the fluid. Ultimately, we extract the missing contributions from the momentum equation in order to estimate the relative strength of the various forcing terms. In this way we are able to make estimates of the plasma beta, as well as predictthe spectral character and total power of Alfvén waves radiated from the supra-arcade region.

Observations of the Formation, Development, and Structure of a Current Sheet in an Eruptive Solar Flare

“…For points in the region surrounding the current sheet, peak temperatures were between 2 and 3MK. Other authors who measured temperatures in this range in the vicinity of current-sheet related phenomena (Hanneman & Reeves 2014;Scott et al 2016) attributed them to foreground and background emission along the line of sight.…”

“…Likewise, in an analysis of similar events, Scott et al (2016) attempted to deduce the magnetic structure of SADs and the plasma that surrounds them. In particular, they attempted to determine the plasma beta, which is the ratio between thermal and magnetic energy density in a plasma.…”

MHD simulations of coronal dark downflows considering thermal conduction

“…We call islands to vortices totally enclosed by magnetic field lines. As in McKenzie (2013) and in Scott et al (2016), initially the fan has an average value of β 1 ∼ 28. With this value of β the magnetic forces do not play a dominant role in the turbulent dynamics.…”

“…The simulation is carried on in two stages. As turbulent fans seem to be the natural site for the development of SADs (McKenzie 2013; Scott et al 2016), the first is the turbulent stage where a turbulent state is obtained from initially homogeneous conditions (second row of Table 1) through the action of a solenoidal driving force. In order to do this we use the method of Federrath et al (2010) (FromFile implementation of the Stir unit in the flash code).…”

MHD Simulations of Coronal Supra-Arcade Downflows Including Anisotropic Thermal Conduction

“…The anti-parallel case defines the position of the CS. As the CS is expected to be a very thin region (∼ 1 Mm) surrounded by the fan itself and vortical features were apparently detected all over the fan region (McKenzie 2013;Scott et al 2016), we assume that eddies must be formed independently of the location of the CS. Thus, we perform the analysis using the more general parallel configuration and we will come back later to the discussion of the anti-parallel configuration.…”