2023ApJ...948...97S

We present new HST/WFC3 optical images of a region in the northeastern part of the Cygnus Loop supernova remnant, which includes a well-studied Balmer-line filament. These data represent the third epoch of HST Hα imaging and a second epoch of [O III] λ5007 imaging of that particular filament. The Hα images were used to measure the proper motions at various locations along the shock front, and the values ranged from 55 to 85 mas yr^–1 with a median value of 70 mas yr^–1, which corresponds to a shock velocity of 240 km s^–1. The proper motions between epochs 1 and 2 were the same as between epochs 2 and 3, implying that there has been no measurable deceleration of the shock in the 22 yr period between the first and third epochs. The range of proper motions (and therefore shock velocities) along the filament indicate variations of over a factor of two in the preshock density. The [O III] emission is prominent toward one end of the filament where the shock has transitioned from nonradiative to radiative. The proper motion is smaller than for the Hα filaments, and it corresponds to a shock velocity of about 155 km s^–1. The images obtained about 18 yr apart show that the [O III] morphology has not changed, which places limits on any short-timescale variations due to catastrophic cooling or thermal instabilities. We find that the effective shock age is less than the eddy turnover timescale, which implies that turbulence has not yet influenced the dynamics of the shock.