A unified view of coronal loop contraction and oscillation in flares

TL;DR

This paper presents a unified model explaining how coronal loop contraction and oscillation are both responses to magnetic energy removal during solar flares, clarifying their relationship and diagnostic potential.

Contribution

It introduces a simple model that unifies the understanding of loop contraction and oscillation phenomena observed during solar flares.

Findings

Loop contractions and oscillations can occur simultaneously as a response to magnetic energy removal.

Oscillations are more likely in higher loops, while shorter loops tend not to oscillate.

Loop motions can serve as diagnostics for magnetic energy changes during flares.

Abstract

Context: Transverse loop oscillations and loop contractions are commonly associated with solar flares, but the two types of motion have traditionally been regarded as separate phenomena. Aims: We present an observation of coronal loops contracting and oscillating following onset of a flare. We aim to explain why both behaviours are seen together and why only some of the loops oscillate. Methods: A time sequence of SDO/AIA 171 \r{A} images is analysed to identify positions of coronal loops following the onset of M6.4 flare SOL2012-03-09T03:53. We focus on five loops in particular, all of which contract during the flare, with three of them oscillating as well. A simple model is then developed for contraction and oscillation of a coronal loop. Results: We propose that coronal loop contractions and oscillations can occur in a single response to removal of magnetic energy from the corona. Our model reproduces the various types of loop motion observed and explains why the highest loops oscillate during their contraction while no oscillation is detected for the shortest contracting loops. The proposed framework suggests that loop motions can be used as a diagnostic for the removal of coronal magnetic energy by flares, while rapid decrease of coronal magnetic energy is a newly-identified excitation mechanism for transverse loop oscillations.