Evidence of solar flare triggering due to loop-loop interaction caused by footpoint shear motion

Abstract

We analyze multi-wavelength data of an M7.9/1N class solar flare which occurred on 2006 April 27 in AR NOAA 10875. GOES soft X-ray images provide the most likely signature of two interacting loops and their reconnection, which triggers the solar flare. TRACE 195Å images also reveal the loop–loop interaction and the formation of “X” points with converging motion (∼30 km s−1) at the reconnection site in between this interacting loop system. This provides evidence of progressive reconnection and flare maximization at the interaction site in the active region. The absence of type III radio bursts during this time period indicates no opening of magnetic field lines during the flare energy release, which implies that the change of field line connectivity/orientation occurred only during the loop–loop interaction and reconnection process. The Ondrejov dynamic radio spectrum shows an intense decimetric (DCIM) radio burst (2.5–4.5 GHz, duration ∼3 minutes) during the flare initiation, which reveals the signature of particle acceleration from the reconnection site during loop–loop interaction. The double-peak structures at 4.9 and 8.8 GHz provide the most likely confirmatory signature of the loop–loop interaction at the flare site in the active region. RHESSI hard X-ray images also show the loop-top and footpoint sources of the corresponding two-loop system, which act like current-carrying flux tubes with resultant opposite magnetic fields and net force of attraction, and their coalescence during the flare maximum. We also suggest that the shear motion/rotation of the footpoint of the smaller loop, which is anchored in the opposite polarity spot, may be responsible for the flare energy buildup and its eventual release due to the loop–loop interaction.