Extreme Red-wing Enhancements of UV Lines during the 2022 March 30 X1.3 Solar Flare

Here, we present the study of a compact emission source during an X1.3 flare on 2022 March 30. Within a similar to 41 s period (17:34:48 UT to 17:35:29 UT), Interface Region Imaging Spectrograph observations show spectral lines of Mg ii, C ii, and Si iv with extremely broadened, asymmetric red wings. This source of interest (SOI) is compact, similar to 1.'' 6, and is located in the wake of a passing ribbon. Two methods were applied to measure the Doppler velocities associated with these red wings: spectral moments and multi-Gaussian fits. The spectral-moments method considers the averaged shift of the lines, which are 85, 125, and 115 km s-1 for the Mg ii, C ii, and Si iv lines respectively. The red-most Gaussian fit suggests a Doppler velocity up to similar to 160 km s-1 in all of the three lines. Downward mass motions with such high speeds are very atypical, with most chromospheric downflows in flares on the order 10-100 km s-1. Furthermore, extreme-UV (EUV) emission is strong within flaring loops connecting two flare ribbons located mainly to the east of the central flare region. The EUV loops that connect the SOI and its counterpart source in the opposite field are much less brightened, indicating that the density and/or temperature is comparatively low. These observations suggest a very fast downflowing plasma in the transition region and upper chromosphere, which decelerates rapidly since there is no equivalently strong shift of the O I chromospheric lines. This unusual observation presents a challenge that models of the solar atmosphere's response to flares must be able to explain.