Evidence for Downflows in the Narrow Plasma Sheet of 2017 September 10 and Their Significance for Flare Reconnection

Abstract

Current sheets are believed to form in the wakes of erupting flux ropes and to enable the magnetic reconnection responsible for an associated flare. Multiwavelength observations of an eruption on 2017 September 10 show a long, linear feature widely taken as evidence of a current sheet viewed edge-on. The relation between the hightemperature, high-density plasma thus observed and any current sheet is not yet entirely clear. We estimate the magnetic field strength surrounding the sheet and conclude that approximately one-third of all flux in the active region was opened by the eruption. Subsequently decreasing field strength suggests that the open flux closed down over the next several hours through reconnection at a rate F 5 10 17 Mx s −1. We find in AIA observations evidence of downward-moving, dark structures analogous to either supra-arcade downflows, more typically observed above flare arcades viewed face-on, or supra-arcade downflowing loops, previously reported in flares viewed in this perspective. These features suggest that the plasma sheet is composed of the magnetic flux retracting after being reconnected high above the arcade. We model flux tube retraction following reconnection to show that this process can generate high densities and temperatures as observed in the plasma sheet. The retracting flux tubes reach their highest temperatures at the end of their retraction, well below the site of reconnection, consistent with previous analysis of AIA and EIS data showing a peak in the plasma temperature near the base of this particular sheet.

Radio Spectroscopic Imaging of a Solar Flare Termination Shock: Split-band Feature as Evidence for Shock Compression

“…One possibility involves plasma flowing within a high-β plasma sheet that has a low Alfvén speed. Another explanation argues that these features may be plasma structures embedded in the current sheet, which are not necessarily the Alfvénic reconnection outflows themselves (see, e.g., discussions in Longcope et al 2018). In any case, we argue that it is highly probable that the reconnection outflows can well exceed the observed speeds of the plasma downflows, and can be supermagnetosonic to drive a fast-mode flare termination shock with a Mach number up to 2.0.…”

Dynamics of Late-stage Reconnection in the 2017 September 10 Solar Flare

“…Multiple studies have revealed evidence for turbulence or tearing mode instabilities within the plasma sheet, such as unusually high emission line widths, and associated nonthermal velocities, which Warren et al (2018) interpreted as reconnection-induced turbulence and outflows. Quasi-periodic pulsations have been reported (Cheng et al 2018;Longcope et al 2018;Hayes et al 2019), along with small-scale velocity fluctuations consistent with turbulent plasmoid fragmentation (Cheng et al 2018). Most of these observations occurred within 40 minutes of flare onset.…”

Spectropolarimetric Insight into Plasma Sheet Dynamics of a Solar Flare

“…Perhaps the brightest and longest-lived plasma-sheet observation to date is associated with an X8.2-class flare on September 10th 2017 (e.g. Long et al 2018;Warren et al 2018;Kuridze et al 2019;Li et al 2018, Cheng et al 2018Longcope et al 2018;Gary et al 2018;Morosan et al 2019). The flare and coronal mass ejection erupted from AR 12673 on the western solar limb, observed across the spectrum by multiple spacebased and ground-based instruments.…”