Institution(s): 1. National Center for Space Weather, China Meteorological Administration
Eruptive flares are one of the most powerful activities on the Sun. In ob- servations, they are often characterized as impulsive energy release followed by flare ribbons and post-flare loops. Post-flare loops are arcade-like loop systems that appear during the gradual phases of eruptive flares. The extreme ultra- violet (EUV) observations from the Atmospheric Imaging Assembly on-board the Solar Dynamics Observatory (SDO) allow us to inves- tigate the fine structures in post-flare loops. This work focuses on the dark structures of post-flare loops during X-class flares, which are seldom analyzed in previous works and termed dark post-flare loops (DPFLs) here. DPFLs during flares are identified in SDO EUV obser- vations and their properties are carefully analyzed for the first time. The results show that: (1) The DPFLs are darker than their surround- ing loops but brighter than the background region. Most of the DPFLs are distinguishable in 171, 193 and 211 ̊A channels and some of them can be found in all EUV channels. (2) We find downflows along the DPFLs have an average velocity of 76 ± 19 km s−1. Multiple downflows are often observed in different loops simultaneously. (3) The average lifetime of the DPFLs is 10.0±5.5 min. The cooling time is 276–852 s and the cooling process is domi- nated by the radiation during the decay phase. Finally, DPFLs are found in all the 20 cases of this work, which suggests they are a common phenomenon during the decay phase of X-class flares.