Evidence For Mixed Helicity in Erupting Filaments

TL;DR

This paper presents observations of erupting filaments with rotations that challenge the typical interpretation, suggesting that filament rotation is influenced by the net helicity of the entire system, including filament barbs.

Contribution

It introduces evidence that filament rotation can be opposite to the expected based on post-eruption arcade helicity, highlighting the role of filament barbs in net helicity.

Findings

Filament rotation can be opposite to the arcade's helicity.

Filament barbs can dominate the system's net helicity.

Filament core helicity may differ from surrounding field.

Abstract

Erupting filaments are sometimes observed to undergo a rotation about the vertical direction as they rise. This rotation of the filament axis is generally interpreted as a conversion of twist into writhe in a kink-unstable magnetic flux rope. Consistent with this interpretation, the rotation is usually found to be clockwise (as viewed from above) if the post-eruption arcade has right-handed helicity, but counterclockwise if it has left-handed helicity. Here, we describe two non--active-region filament events recorded with the Extreme-Ultraviolet Imaging Telescope (EIT) on the {\it Solar and Heliospheric Observatory} ({\it SOHO}), in which the sense of rotation appears to be opposite to that expected from the helicity of the post-event arcade. Based on these observations, we suggest that the rotation of the filament axis is in general determined by the net helicity of the erupting system, and that the axially aligned core of the filament can have the opposite helicity sign to the surrounding field. In most cases, the surrounding field provides the main contribution to the net helicity. In the events reported here, however, the helicity associated with the filament ``barbs'' is opposite in sign to and dominates that of the overlying arcade.