A Circular-ribbon Solar Flare Following an Asymmetric Filament Eruption

TL;DR

This paper analyzes a specific X1.0 solar flare event, revealing how asymmetric filament eruption and magnetic reconnection produce a circular flare ribbon, enhancing understanding of solar magnetic field dynamics during flares.

Contribution

It presents detailed observations linking filament eruption asymmetry with circular flare ribbon formation, supported by a combined analysis of EUV imaging and magnetic field modeling.

Findings

Asymmetric filament eruption linked to circular ribbon formation.

Filament eruption driven by MHD instability and magnetic reconnection.

Sequential brightening of the circular ribbon observed during eruption.

Abstract

The dynamic properties of flare ribbons and the often associated filament eruptions can provide crucial information on the flaring coronal magnetic field. This Letter analyzes the GOES-class X1.0 flare on 2014 March 29 (SOL2014-03-29T17:48), in which we found an asymmetric eruption of a sigmoidal filament and an ensuing circular flare ribbon. Initially both EUV images and a preflare nonlinear force-free field model show that the filament is embedded in magnetic fields with a fan-spine-like structure. In the first phase, which is defined by a weak but still increasing X-ray emission, the western portion of the sigmoidal filament arches upward and then remains quasi-static for about five minutes. The western fan-like and the outer spine-like fields display an ascending motion, and several associated ribbons begin to brighten. Also found is a bright EUV flow that streams down along the eastern fan-like field. In the second phase that includes the main peak of hard X-ray (HXR) emission, the filament erupts, leaving behind two major HXR sources formed around its central dip portion and a circular ribbon brightened sequentially. The expanding western fan-like field interacts intensively with the outer spine-like field, as clearly seen in running difference EUV images. We discuss these observations in favor of a scenario where the asymmetric eruption of the sigmoidal filament is initiated due to an MHD instability and further facilitated by reconnection at a quasi-null in corona; the latter is in turn enhanced by the filament eruption and subsequently produces the circular flare ribbon.