Spectroscopic Observations of High-speed Downflows in a C1.7 Solar Flare

TL;DR

This study analyzes high-resolution UV spectra of a C1.7 solar flare, revealing high-speed downflows in cool lines during the rise phase, likely caused by magnetic reconnection, with some regions showing gentler redshifts from different processes.

Contribution

First detailed spectroscopic analysis of high-speed downflows in a small solar flare using IRIS data, linking redshifts to magnetic reconnection in the transition region.

Findings

Redshifts up to 150 km/s observed in cool lines during flare rise.

Redshifts correlate with magnetic reconnection events.

Different regions show distinct redshift features, indicating multiple physical processes.

Abstract

In this paper, we analyze the high-resolution UV spectra for a C1.7 solar flare (SOL2017-09-09T06:51) observed by the \textit{Interface Region Imaging Spectrograph} (\textit{IRIS}). {We focus on the spectroscopic observations at the locations where the cool lines of \ion{Si}{4} 1402.8 \AA\ ($\sim$10$^{4.8}$ K) and \ion{C}{2} 1334.5/1335.7 \AA\ ($\sim$10$^{4.4}$ K) reveal significant redshifts with Doppler velocities up to $\sim$150 km s$^{-1}$.} These redshifts appear in the rise phase of the flare, then increase rapidly, reach the maximum in a few minutes, and proceed into the decay phase. Combining the images from \textit{IRIS} and Atmospheric Imaging Assembly (AIA) on board the {\em Solar Dynamics Observatory} ({\em SDO}), we propose that the redshifts in the cool lines are caused by the downflows in the transition region and upper chromospheric layers, which likely result from a magnetic reconnection leading to the flare. In addition, the cool \ion{Si}{4} and \ion{C}{2} lines show gentle redshifts (a few tens of km s$^{-1}$) at some other locations, which manifest some distinct features from the above locations. This is supposed to originate from a different physical process.