Elongation of Flare Ribbons

TL;DR

This paper analyzes the motion and shear of flare ribbons and loops during two-ribbon solar flares, revealing how their elongation and shear decrease as the flare evolves, providing insights into magnetic reconnection.

Contribution

It introduces a detailed analysis of flare ribbon elongation and shear, linking ribbon motion to magnetic field properties in the corona.

Findings

Flare ribbons and loops spread along the PIL at speeds of a few to a hundred km/s.

Shear decreases toward a potential field as the flare progresses.

Fast bi-directional elongation correlates with strong shear and a large magnetic guide field.

Abstract

We present an analysis of the apparent elongation motion of flare ribbons along the polarity inversion line (PIL) as well as the shear of flare loops in several two-ribbon flares. Flare ribbons and loops spread along the PIL at a speed ranging from a few to a hundred km/s. The shear measured from conjugate foot-points is consistent with the measurement from flare loops, and both show the decrease of shear toward a potential field as a flare evolves and ribbons and loops spread along the PIL. Flares exhibiting fast bi-directional elongation appear to have a strong shear, which may indicate a large magnetic guide field relative to the reconnection field in the coronal current sheet. We discuss how the analysis of ribbon motion could help infer properties in the corona where reconnection takes place.