Spectroscopic Observations of High-speed Downflows in a C1.7 Solar Flare
Abstract
In this paper, we analyze the high-resolution UV spectra for a C1.7 solar flare (SOL2017-09-09T06:51) observed by the Interface Region Imaging Spectrograph (IRIS). We focus on the spectroscopic observations at the locations where the cool lines of Si4 1402.8 \ (104.8 K) and C2 1334.5/1335.7 \ (104.4 K) reveal significant redshifts with Doppler velocities up to 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 IRIS and Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory ( 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 Si4 and C2 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.