Slow Rise and Partial Eruption of a Double-Decker Filament. I Observations and Interpretation

TL;DR

This study investigates the evolution and eruption of a double-decker solar filament, revealing how magnetic flux transfer and instability mechanisms contribute to its partial eruption and associated flare phenomena.

Contribution

It provides new insights into the magnetic flux transfer process and the role of instabilities in the eruption of a double-decker filament, supported by detailed observations and interpretation.

Findings

Filament threads intermittently brighten and merge, increasing the upper branch's flux.

Flux transfer may lead to loss of equilibrium and eruption.

Eruption involves writhe-twist-writhe helicity transfer, excluding helical kink instability as trigger.

Abstract

We study an active-region dextral filament which was composed of two branches separated in height by about 13 Mm. This "double-decker" configuration sustained for days before the upper branch erupted with a GOES-class M1.0 flare on 2010 August 7. Analyzing this evolution, we obtain the following main results. 1) During hours before the eruption, filament threads within the lower branch were observed to intermittently brighten up, lift upward, and then merge with the upper branch. The merging process contributed magnetic flux and current to the upper branch, resulting in its quasi-static ascent. 2) This transfer might serve as the key mechanism for the upper branch to lose equilibrium by reaching the limiting flux that can be stably held down by the overlying field or by reaching the threshold of the torus instability. 3) The erupting branch first straightened from a reverse S shape that followed the polarity inversion line and then writhed into a forward S shape. This shows a transfer of left-handed helicity in a sequence of writhe-twist-writhe. The fact that the initial writhe is converted into the twist of the flux rope excludes the helical kink instability as the trigger process of the eruption, but supports the occurrence of the instability in the main phase, which is indeed indicated by the very strong writhing motion. 4) A hard X-ray sigmoid, likely of coronal origin, formed in the gap between the two original filament branches in the impulsive phase of the associated flare. This supports a model of transient sigmoids forming in the vertical flare current sheet. 5) Left-handed magnetic helicity is inferred for both branches of the dextral filament. 6) Two types of force-free magnetic configurations are compatible with the data, a double flux rope equilibrium and a single flux rope situated above a loop arcade.