Early Evolution of An Energetic Coronal Mass Ejection

Abstract:

We study a coronal mass ejection (CME) associated with an X-class flare, whose initiation is clearly observed in low corona with high-cadence, high-resolution EUV images, providing us a rare opportunity to witness the early evolution of an energetic CME in detail. The eruption starts with a slow expansion of cool overlying loops (~1 MK), below which a reverse S-shaped dimming is seen immediately above the brightening active region in hot EUV passbands. The dimming is associated with a rising hot arch (~6 MK), which we interpret as a preexistent, high-lying flux rope. This is followed by the arising of a double hot channel (DHC; ~10 MK) from the core of the active region. The higher structures rise earlier and faster than lower ones, with the leading front undergoing extremely rapid acceleration up to 35 km/s^2. This suggests that the torus instability is the major triggering mechanism and that it is the high-lying flux rope rather than the DHC that drives the eruption. A global EUV wave, which is closely associated with a metric type II radio burst, initiates as the laterally under-expanding loop system pushes down neighboring coronal loops that subsequently oscillate vertically. Polar plumes oscillate horizontally with the wave passing through at ~400 km/s. A local EUV wave bounded by the expanding loop system propagates first sunward and then outward at 200 km/s. The observation suggests that over-expansion is not necessary for EUV wave excitation.