Spectroscopic Observations of Current Sheet Formation and Evolution

Abstract

We report on the structure and evolution of a current sheet that formed in the wake of an eruptive X8.3 flare observed at the west limb of the Sun on September 10, 2017. Using observations from the EUV Imaging Spectrometer (EIS) on Hinode and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), we find that plasma in the current sheet reaches temperatures of about 20 MK and that the range of temperatures is relatively narrow. The highest temperatures occur at the base of the current sheet, in the region near the top of the post-flare loop arcade. The broadest high temperature line profiles, in contrast, occur at the largest observed heights. Further, line broadening is strong very early in the flare and diminishes over time. The current sheet can be observed in the AIA 211 and 171 channels, which have a considerable contribution from thermal bremsstrahlung at flare temperatures. Comparisons of the emission measure in these channels with other EIS wavelengths and AIA channels dominated by Fe line emission indicate a coronal composition and suggest that the current sheet is formed by the heating of plasma already in the corona. Taken together, these observations suggest that some flare heating occurs in the current sheet while additional energy is released as newly reconnected field lines relax and become more dipolar.

Spectropolarimetric Insight into Plasma Sheet Dynamics of a Solar Flare

“…Important earlier studies of intensity images and spectra include that of Warren et al (2018). They used Extreme ultraviolet (EUV) Imaging Spectrometer (EIS) (Culhane et al 2007) and Atmospheric Imaging Assembly (AIA) (Lemen et al 2012) data to study the spectroscopic evolution and structure of the plasma-sheet.…”

“…It was deduced that the plasma-sheet must be heated by processes originating from magnetic reconnection, as is consistent with the CSHKP model. Li et al (2018) and Warren et al (2018) investigated non-thermal broadening of spectral lines within the plasma-sheet, finding non-thermal velocities as high as 200 km s −1 . The highest line widths (measuring velocities of plasma superposed along the line-of-sight, or "LOS") were seen first at the base of the plasmasheet, later they shifted to higher altitudes.…”

“…Given the plasma-sheet's high temperature, most of the observed emission is likely from the 20 MK Fe XXIV line. Despite its high temperature, the plasmasheet is also seen in cooler AIA passbands, such as 211 A (Warren et al (2018)), dominated by plasma closer to 2 MK. In AIA 193Å, the plasma-sheet is clearly visible GOES−15 X−ray Flux 15:00 18:00 21:00 00:00 03:00…”

A Compact Source for Quasi-periodic Pulsation in an M-class Solar Flare

“…Moreover, the dynamics of a current sheet, which is a direct byproduct of magnetic reconnection, does not normally reveal itself in the EUV emissions of coronal plasma. Albeit difficult, several studies attempted to estimate the magnetic field or current sheet with lucky viewing angles in imaging or spectroscopic measurement (Su et al 2013;Longcope et al 2018;Warren et al 2018). To investigate the physics of QPP, one has to seek for signature of repetitive waves and flows during magnetic reconnection (McLaughlin et al 2012).…”

Drifting Pulsation Structure at the Very Beginning of the 2017 September 10 Limb Flare

“…Note that these plasma densities are in the interval between the densities estimated in the flare arcade at 15:59 UT, i.e. at about 10 minutes after DPS, (0.9-2.0 × 10 11 cm −3 (Polito et al 2018)) and the plasma density in the current sheet (10 10 cm −3 (Warren et al 2018)). Because the density in the arcade increases during the flare and the density in the DPS source is expected to be similar to that in the current sheet, the estimated densities support our interpretation.…”