  Supra-arcade downflows (SADs) are dark voids descending towards the
post-reconnection flare loops and exhibit obvious variation in properties like
width. However, due to the lack of further statistical studies, the mechanism
behind such variations hitherto remains elusive. Here we statistically
investigated widths of 81 SADs observed in one flare by the Solar Dynamics
Observatory (SDO). For each of SADs, six moments were selected with equal time
intervals to measure their widths at different stages of their evolution. It is
found that most SADs show a roughly monotonous width decrease during their
descents, while some SADs with small initial widths can have complex
evolutions. 3D reconstruction results based on SDO and Solar Terrestrial
Relations Observatory Ahead (STEREO-A) images and thermal properties analysis
reveal that differences in magnetic and plasma environments may result in that
SADs in the north are overall wider than those in the south. Additionally,
correlation analysis between the width and other parameters of SADs was further
conducted and revealed that: (1) SADs with different initial widths show no
significant differences in their temperature and density evolution
characteristics; (2) SADs with small initial widths usually appear in lower
heights, where more frequent collisions between SADs could lead to their
intermittent acceleration, width increment, and curved trajectories. These
results indicate that SADs with different initial widths are produced the same
way while different environments (magnetic field or plasma) could affect their
subsequent width evolutions.

Solar and Stellar Astrophysics