Authors: Yan Xu (NJIT), Graham S. Kerr (NASA, CUA), Vanessa Polito (BAERI, Lockheed Martin), Nengyi Huang (NJIT), Ju Jing (NJIT), Haimin Wang (NJIT)

On 2022-March-30, an X1.3 flare occurred in AR 12975, peaked around 17:20 UT. During a short period (17:34:51 UT to 17:35:27 UT), IRIS observations show transient emissions with strongly enhanced red-wings, in Mg II, C II and Si IV. The source of interest (SOI) is located behind the brightest area of the flare ribbon, with a characteristic size about 1′′.6. The average red-shifts are 85 km/s, 125 km/s and 114 km/s for the Mg II, C II and Si IV lines respectively, using the spectral moment method The red-most component in the multi-Gaussian fit suggests a Doppler velocity around 160 km/s in all of the three lines. Downward mass motions with such high speeds are
very atypical, with most chromospheric downflows on the order 10-100 km/s. These observations suggest a dense, very fast downflowing plasma in transition region and upper chromospher. This unusual observation presents a challenge to models of the solar atmosphere’s response to flares.