Solar post-flare loops (PFLs) are arcade-like loop systems that appear during the gradual phases of eruptive flares. The extreme ultraviolet (EUV) observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) allow us to investigate the fine structures in PFLs. In this work, we focus on studying the dark post-flare loops (DPFLs) during X-class flares, which are more evident in SDO/AIA data than in previous EUV data. We identify and analyze the DPFLs observed by SDO and find that: (1) the DPFLs of an X5.4 flare have an average lifetime of 10.0 ?±? 5.5 minutes, an average width of 1022 ?±? 339 km, and an average maximum length of 33 ?±? 10 Mm, (2) blob-like falling features with a size close to the resolution of SDO/AIA are identified in the DPFLs and have an average velocity of 76 ?±? 19 km s?1, and (3) the average widths of the DPFLs slightly increase with the characteristic temperatures in the AIA 304, 171, 193, and 211 ? channels. Our investigation shows that DPFLs are found in all of the 20 cases within this study, which suggests that they are a common phenomenon in X-class flares and are probably produced by the same mechanism that creates coronal rain.
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