  In this paper, we present multiwavelength observations of helical kink
instability as a trigger of a CME which occurred in AR NOAA 11163 on 24
February 2011. The CME was associated with a M3.5 limb flare. High resolution
observations from SDO/AIA suggest the development of helical kink instability
in the erupting prominence, which implies a flux rope structure of the magnetic
field. A brightening starts below the apex of the prominence with its slow
rising motion (~100 km/s) during the activation phase. A bright structure,
indicative of a helix with ~3-4 turns, was transiently formed at this position.
The corresponding twist of ~$6\pi-8\pi$ is sufficient to generate the helical
kink instability in a flux rope according to recently developed models. A
slowly rising blob structure was subsequently formed at the apex of the
prominence, and a flaring loop was observed near the footpoints. Within two
minutes, a second blob was formed in the northern prominence leg. The second
blob erupts (like a plasmoid ejection) with the detachment of the northern
prominence leg, and flare intensity maximizes. The first blob at the prominence
apex shows rotational motion in the counterclockwise direction in the plane of
sky, interpreted as unwinding motion of a helix, and it also erupts to give the
coronal mass ejection (CME). RHESSI hard X-ray sources show the two footpoint
sources and a loop-top source during the flare. We found RHESSI hard X-ray
flux, soft X-ray flux derivative and CME acceleration in the low corona
correlate well, which is in agreement with the standard flare model (CSHKP). We
also discuss the possible role of ballooning as well as torus instabilities in
driving the CME. We conclude that the CME and flare were triggered by the
helical kink instability in a flux rope and accelerated mainly by the torus
instability.
