Active region transient brightenings

This paper reports on a qualitative study on the weakest flarelike brightenings in active region that can be observed with current coronal imagers. Specificallly, we investigate the correspondence of the "active region transient brightenings"(ARTB) first observed almost a decade ago with SXT with similar brightenings in the EUV that are now observed by EIT and TRACE. For this goal, exceptionally high cadence image sequences were acquired of a small but rapidly evolving active region (NOAA 8218), simultaneously by SXT, EIT, and TRACE. Within the timeframe of this coordinated campaign, we detected 41 soft X-ray brightenings and 373 EUV brightenings. We find that the strongest brightenings observed by EIT are indeed the EUV counterparts of the ARTBs seen by SXT. Weaker brightenings seen by EIT often do not have an X-ray counterpart. Among the brightenings detected with SXT we discover a new subpopulation, consisting of events that brighten in soft X-rays only, at a footpoint of a pre-existing SXT loop shortly after an ARTB occurred at the other footpoint. The propagation speed of the perturbation suggests an interpretation in terms of slow mode MHD waves