A unified view of coronal loop contraction and oscillation in 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 that contract and oscillate following the 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 Å images is analysed to identify the positions of coronal loops following the onset of the 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 the 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 in coronal magnetic energy is a newly identified excitation mechanism for transverse loop oscillations.

Quasi-periodic Pulsations in Solar and Stellar Flares: An Overview of Recent Results (Invited Review)

“…This is an observational indication of the nature of the QPP, which is interacting directly with (some of) the surrounding loops. This effect (as also suggested by Zimovets and Nakariakov, 2015) was observed and modelled by Russell, Simões, and Fletcher (2015) for five contracting loops during a solar flare. The authors found that the highest loops oscillated during their contraction while the shortest loops did not.…”

Seismology of contracting and expanding coronal loops using damping of kink oscillations by mode coupling

“…Finally, in the intermediate case where the time scales are the same order of magnitude, oscillation and displacement occur in superposition as two parts of a single response. The different behaviours can be found in the same active region, ordered by oscillation period, as in the event studied by Simões et al (2013) and Russell et al (2015). More generally, comparison of observed implosion time scales (typically several minutes) with reported periods of transverse loop oscillations (from less than two minutes to longer than thirty minutes) suggests that transverse loop oscillations of contracting loops should be a common feature of implosions, with the caveat that Russell et al (2015) also predicted that the largest amplitude oscillations require that the change in the equilibrium position is initially sharp.…”

“…The different behaviours can be found in the same active region, ordered by oscillation period, as in the event studied by Simões et al (2013) and Russell et al (2015). More generally, comparison of observed implosion time scales (typically several minutes) with reported periods of transverse loop oscillations (from less than two minutes to longer than thirty minutes) suggests that transverse loop oscillations of contracting loops should be a common feature of implosions, with the caveat that Russell et al (2015) also predicted that the largest amplitude oscillations require that the change in the equilibrium position is initially sharp. Implosions are potentially valuable as a source of transverse loop oscillations because they usually excite multiple loops, which raises the prospect of using them to build up a picture of properties across the active region periphery, and the opportunity to learn how loop properties evolve during the displacement.…”

“…Loop oscillations were also noted in the implosions studied by Liu & Wang (2010); Sun et al (2012); Gosain (2012) and Simões et al (2013), which led Russell et al (2015) to consider the possibility of a connection between these phenomena. Russell et al (2015) concluded that when a loop's environment changes in such a way as to change the location or shape in which the loop is in equilibrium, the loop responds in one of three ways (summarised in their Fig. 4).…”

“…Therefore, peripheral loops must move inwards to dynamically maintain or restore force balance. Longer presentations of the energy-pressure argument and how it applies to confined flares and eruptions can be found in the papers by Janse & Low (2007); Liu et al (2012); Shen et al (2014);Russell et al (2015) and Wang et al (2016), with Fig. 3 of Russell et al (2015) illustrating the basic concept.…”