Coronal Mass Ejections Associated With Impulsive Solar Flares - Observations With SECCHI EUVI On STEREO

Long-duration flares, sometimes referred to as Long Decay Events (LDEs), are known to be unmistakable signatures of coronal mass ejections (CMEs), and often of fast and large ones. Short-duration or impulsive flares, on the other hand, do not as frequently accompany CMEs, even though X-ray plasmoid ejections seen in some of these flares may suggest that all flares are eruptive irrespective of durations. Some of these ejections in X-ray or EUV images could be failed ejections, however, meaning that they do not move into interplanetary medium. A complementary, and perhaps more reliable signature of a CME in the low corona may be large-scale dimming typically observed at 1-2 MK. We report on high cadence observations of SECCHI EUVI on STEREO that show this phenomenon in weak impulsive flares more frequently than expected. We systematically study flare periods with good data coverage. In order to avoid false dimming, we use both base and running difference images after carefully co-aligning the image pairs. Some of the dimming events were observed in more than one channel and at two widely separated view angles, letting us better understand the nature of dimming especially in terms of the associated CME. We discuss how the properties of dimming are reflected in CME parameters, how to distinguish the impulsive flares with large- scale effects from those that are confined, and whether similar events could account for orphan ICMEs without a clearly associated CME near the Sun.