Shock waves, as evidenced by type II radio bursts, often accompany flares and coronal mass ejection transients. At present, the density enhancements observed by coronagraphs are believed by some to be ejected matter from the low corona, and by others to be the compressed material behind a shock front. If the former is correct, one would expect in some cases to see a density enhancement, associated with the compression region of the shock, some distance ahead of the transient ejecta. Such a density enhancement has not been previously reported. The coronal transient of 1980 June 29 (0233 UT) was observed with the High Altitude Observatory's Coronagraph/Polarimeter aboard SMM. This flare-associated coronal transient event was well observed with the Culgoora Radioheliograph, including a well-developed type II burst. Visible on the coronagraph images is a faint circular arc moving out well ahead of the transient loops. This arc is moving at more than 900 km s-1 while the transient itself is moving at a speed of about 600 km s-1. Both the arc and transient appear to have originated either prior to the X-ray flare or at some height above the flare at the time of the flare. The type II burst observed at Culgoora is associated with the transient loops, and no type II emission is identified with the faint arc. Due to its great speed, we interpret the faint arc as a manifestation of a shock wave, but also envision a separate shock wave associated with the transient loops as evidenced by the type II emission. Preliminary density measurements are consistent with this interpretation, and show the outer shock wave associated with the faint arc to have a Mach number MA ≤ 1.7. At present we have no convincing explanation for the lack of a type II burst in association with the arc. This work was supported in part by NASA through grants NSG-7287 and NAGW-91 to the University of Colorado, Boulder, and S-55989 to the High Altitude Observatory, National Center for Atmospheric Research. The National Center for Atmospheric Research, NCAR, is sponsored by the National Science Foundation.
All Science Journal Classification (ASJC) codes
- Aerospace Engineering
- Astronomy and Astrophysics
- Atmospheric Science
- Space and Planetary Science
- Earth and Planetary Sciences(all)