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

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 Si iv 1402.8 Å (∼10^4.8 K) and C ii 1334.5/1335.7 Å (∼10^4.4 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 on board the Solar Dynamics Observatory, 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 Si iv and C ii 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.