Evidence of Solar Flare Triggering due to Loop-Loop Interaction Caused by Footpoint Shear-Motion

TL;DR

This study provides multi-wavelength observational evidence that loop-loop interactions caused by footpoint shear motion can trigger solar flares, highlighting the role of magnetic reconnection and particle acceleration.

Contribution

It offers detailed observational confirmation of loop-loop interaction as a trigger mechanism for solar flares, emphasizing the role of footpoint shear motion and reconnection signatures.

Findings

Reconnection signatures observed in X-ray and radio data.

Absence of type III radio burst indicates no open field lines.

Particle acceleration evidenced by intense decimetric radio burst.

Abstract

We analyze multi-wavelength data of a M7.9/1N class solar flare which occurred on 27 April, 2006 from 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 195 A images also reveal the loop-loop interaction and the formation of `X' points with converging motion (~30 km/s) at the reconnection site in-between this interacting loop system. This provides the evidence of progressive reconnection and flare maximization at the interaction site in the active region. The absence of type III radio burst during this time period indicates no opening of magnetic field lines during the flare energy release, which implies only the change of field lines connectivity/orientation 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 min) during 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 and their coalescence during the flare maximum, which act like the current carrying flux-tubes with resultant opposite magnetic fields and the net force of attraction. 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 then its release due to the loop-loop interaction.