Saddle-shaped solar flare arcades

TL;DR

This study analyzes the morphology of solar flare arcades, revealing a common saddle shape across different flare types and environments, linked to specific magnetic reconnection processes and ribbon structures.

Contribution

It identifies a universal saddle-shaped morphology in flare arcades and links it to magnetic reconnection and ribbon hook structures, providing new insights into flare loop formation.

Findings

Saddle-shaped flare arcades are common across different flare classes.

Longer, higher, and inclined loops ('cantles') are key features at arcade ends.

Magnetic reconnection explains the formation of these saddle structures.

Abstract

Arcades of flare loops form as a consequence of magnetic reconnection powering solar flares and eruptions. We analyse the morphology and evolution of flare arcades that formed during five well-known eruptive flares. We show that the arcades have a common saddle-like shape. The saddles occur despite the fact that the flares were of different classes (C to X), occurred in different magnetic environments, and were observed in various projections. The saddles are related to the presence of longer, relatively-higher, and inclined flare loops, consistently observed at the ends of the arcades, which we term `cantles'. Our observations indicate that cantles typically join straight portions of flare ribbons with hooked extensions of the conjugate ribbons. The origin of the cantles is investigated in stereoscopic observations of the 2011 May 9 eruptive flare carried out by the Atmospheric Imaging Assembly (AIA) and Extreme Ultraviolet Imager (EUVI). The mutual separation of the instruments led to ideal observational conditions allowing for simultaneous analysis of the evolving cantle and the underlying ribbon hook. Based on our analysis we suggest that the formation of one of the cantles can be explained by magnetic reconnection between the erupting structure and its overlying arcades. We propose that the morphology of flare arcades can provide information about the reconnection geometries in which the individual flare loops originate.