Imaging and spectroscopic observations of magnetic reconnection and chromospheric evaporation in a solar flare

TL;DR

This study uses IRIS and SDO observations to detect and analyze magnetic reconnection outflows and chromospheric evaporation in a solar flare, providing rare spectroscopic evidence of downflows and detailed plasma dynamics.

Contribution

First spectroscopic detection of significant redshifted Fe XXI emission at a flare reconnection site, revealing downward reconnection outflows and chromospheric evaporation signatures.

Findings

Detection of ~125 km/s redshifted Fe XXI emission at the reconnection site.

Observation of blueshifted Fe XXI at loop footpoints indicating chromospheric evaporation.

Identification of downward-propagating cool plasma blobs along reconnected loops.

Abstract

Magnetic reconnection is believed to be the dominant energy release mechanism in solar flares. The standard flare model predicts both downward and upward outflow plasmas with speeds close to the coronal Alfv\'{e}n speed. Yet, spectroscopic observations of such outflows, especially the downflows, are extremely rare. With observations of the newly launched Interface Region Imaging Spectrograph (IRIS), we report the detection of greatly redshifted ($\sim$125 km s$^{-1}$ along line of sight) Fe {\sc{xxi}} 1354.08\AA{} emission line with a $\sim$100 km s$^{-1}$ nonthermal width at the reconnection site of a flare. The redshifted Fe {\sc{xxi}} feature coincides spatially with the loop-top X-Ray source observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We interpret this large redshift as the signature of downward-moving reconnection outflow/hot retracting loops. Imaging observations from both IRIS and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) also reveal the eruption and reconnection processes. Fast downward-propagating blobs along these loops are also found from cool emission lines (e.g., Si {\sc{iv}}, O {\sc{iv}}, C {\sc{ii}}, Mg {\sc{ii}}) and images of AIA and IRIS. Furthermore, the entire Fe {\sc{xxi}} line is blueshifted by $\sim$260 km s$^{-1}$ at the loop footpoints, where the cool lines mentioned above all exhibit obvious redshift, a result that is consistent with the scenario of chromospheric evaporation induced by downward-propagating nonthermal electrons from the reconnection site.