An Unorthodox X-Class Long-Duration Confined Flare

TL;DR

This paper presents a detailed observation and analysis of a confined X-class long-duration solar flare, revealing unique magnetic structures and plasma temperature distributions that challenge typical flare models.

Contribution

It provides new insights into the magnetic topology and plasma behavior of a confined long-duration flare, highlighting the role of flux emergence and hyperbolic flux tubes.

Findings

The flare's cusp-shaped structure consists of multiple nested loops rising repeatedly.

Temperature analysis shows higher temperatures at the top of the arcade, opposite to typical flares.

Magnetic modeling indicates the involvement of sheared arcades and a flux rope confined by strong magnetic fields.

Abstract

We report the observation of an X-class long-duration flare which is clearly confined. It appears as a compact-loop flare in the traditional EUV passbands (171 and 195 {\AA}), but in the passbands sensitive to flare plasmas (94 and 131 {\AA}), it exhibits a cusp-shaped structure above an arcade of loops like other long-duration events. Inspecting images in a running difference approach, we find that the seemingly diffuse, quasi-static cusp-shaped structure consists of multiple nested loops that repeatedly rise upward and disappear approaching the cusp edge. Over the gradual phase, we detect numerous episodes of loop rising, each lasting minutes. A differential emission measure analysis reveals that the temperature is highest at the top of the arcade and becomes cooler at higher altitudes within the cusp-shaped structure, contrary to typical long-duration flares. With a nonlinear force-free model, our analysis shows that the event mainly involves two adjacent sheared arcades separated by a T-type hyperbolic flux tube (HFT). One of the arcades harbors a magnetic flux rope, which is identified with a filament that survives the flare owing to the strong confining field. We conclude that a new emergence of magnetic flux in the other arcade triggers the flare, while the preexisting HFT and flux rope dictate the structure and dynamics of the flare loops and ribbons during the long-lasting decay phase, and that a quasi-separatrix layer high above the HFT could account for the cusp-shaped structure.