Circular Ribbon Flares and Homologous Jets

TL;DR

This study reports five instances of circular ribbon solar flares with associated surges, revealing complex 3D magnetic reconnection processes involving fan-spine topology and homologous jets, advancing understanding of solar flare dynamics.

Contribution

The paper presents detailed high-resolution observations of circular ribbon flares and homologous jets, highlighting their sequential brightening and magnetic topology, which are novel insights into 3D magnetic reconnection.

Findings

Circular ribbons form around parasitic magnetic fields with circular PILs.

Sequential brightening of ribbons and co-spatial surges observed.

Remote brightenings linked to magnetic flux cancellation.

Abstract

Solar flare emissions in the chromosphere often appear as elongated ribbons on both sides of the magnetic polarity inversion line (PIL), which has been regarded as evidence of a typical configuration of magnetic reconnection. However, flares having a circular ribbon have rarely been reported, although it is expected in the fan--spine magnetic topology involving reconnection at a three-dimensional (3D) coronal null point. We present five circular ribbon flares with associated surges, using high-resolution and high-cadence halpha blue wing observations obtained from the recently digitized films of Big Bear Solar Observatory. In all the events, a central parasitic magnetic field is encompassed by the opposite polarity, forming a circular PIL traced by filament material. Consequently, a flare kernel at the center is surrounded by a circular flare ribbon. The four homologous jet-related flares on 1991 March 17 and 18 are of particular interest, as (1) the circular ribbons brighten sequentially, with co-spatial surges, rather than simultaneously, (2) the central flare kernels show an intriguing "round-trip" motion and become elongated, and (3) remote brightenings occur at a region with the same magnetic polarity as the central parasitic field and are co-temporal with a separate phase of flare emissions. In another flare on 1991 February 25, the circular flare emission and surge activity occur successively, and the event could be associated with magnetic flux cancellation across the circular PIL. We discuss the implications of these observations combining circular flare ribbons, homologous jets, and remote brightenings for understanding the dynamics of 3D magnetic restructuring.