Observations of an X-shaped Ribbon Flare in the Sun and Its Three-dimensional Magnetic Reconnection

TL;DR

This paper presents detailed observations of an unusual X-shaped solar flare ribbon, providing new evidence for three-dimensional magnetic reconnection involving a separator and complex magnetic topology.

Contribution

It offers the first high-resolution observational evidence of 3D magnetic reconnection at a separator during an X-shaped flare ribbon event.

Findings

Revealed four chromospheric flare ribbons forming an X-shape.

Observed the inward and outward spreading of flare brightening along ribbons.

Confirmed the reconnection morphology with coronal loop observations.

Abstract

We report evolution of an atypical X-shaped flare ribbon which provides novel observational evidence of three-dimensional (3D) magnetic reconnection at a separator. The flare occurred on 2014 November 9. High-resolution slit-jaw 1330 A images from the Interface Region Imaging Spectrograph reveal four chromospheric flare ribbons that converge and form an X-shape. Flare brightening in the upper chromosphere spreads along the ribbons toward the center of the "X" (the X-point), and then spreads outward in a direction more perpendicular to the ribbons. These four ribbons are located in a quadrupolar magnetic field. Reconstruction of magnetic topology in the active region suggests the presence of a separator connecting to the X-point outlined by the ribbons. The inward motion of flare ribbons in the early stage therefore indicates 3D magnetic reconnection between two sets of non-coplanar loops that approach laterally, and reconnection proceeds downward along a section of vertical current sheet. Coronal loops are also observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory confirming the reconnection morphology illustrated by ribbon evolution.