Thermal Properties of Current Sheet Plasmas in Solar Flares

TL;DR

This study investigates the thermal properties of plasma sheets in solar flares using EUV observations, revealing that these sheets are generally isothermal with steady temperatures, indicating balanced heating and cooling during reconnection.

Contribution

It provides new insights into the thermal structure of current sheets in solar flares, showing they are typically isothermal and steady, which supports a model of balanced heating and cooling processes.

Findings

Plasma sheets exhibit high, narrow temperature ranges, suggesting isothermal features.

Temperature remains constant at different heights in most cases, with one exception.

Sheet emissions decrease exponentially above the flare arcade, but thicknesses show no height dependence.

Abstract

The current sheet is an essential feature in solar flares and is the primary site for magnetic reconnetion and energy release. Imaging observations feature a long linear structure above the candle-flame-shaped flare loops, which resembles the standard flare model with the current sheet viewed edge-on. We investigate the thermal properties of plasmas surrounding the linear sheet during flares, using EUV observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). The differential emission measure (DEM) analyses show evidence of high temperatures in the plasma sheets (PSs), containing hot emissions from only a narrow temperature range, suggestive of an isothermal feature. The sheet's temperature remains constant at different heights above the flare arcade, peaking at around logT=7.0-7.1; while the well-studied 2017 September 10 X8.2 flare exhibits as an exception in that the temperature decreases with an increasing height and peaks higher (logT=7.25) during the gradual phase. Most PS cases also hold similar emission measures and thicknesses; while the PS's emissions drop exponentially above the flare arcade, the sheet thicknesses show no significant height association as for all the measurements. The characteristics of isothermal and steady temperature suggests balanced heating and cooling processes along the current sheet, particularly additional heating may exist to compensate for the conductive and radiative cooling away from the reconenction site. Our results suggest a steady and uniform sheet structure in the macroscopic scale that results from flare reconnection.