Counter-Streaming Velocities in a Quiescent Filament

TL;DR

This study characterizes counter-streaming flows in solar filaments using high-resolution spectral observations, revealing supersonic velocities and suggesting oscillations rather than magnetic field-aligned flows as their cause.

Contribution

It provides detailed spectroscopic evidence of counter-streaming flows in filaments and proposes a new interpretation involving kink oscillations instead of magnetic alignment.

Findings

Counter-streaming flows are detected in both H-alpha and Mg II lines.

Some filament plasma moves at approximately 20 km/s, indicating supersonic speeds.

Counter-streaming may result from kink transverse oscillations rather than magnetic field-aligned flows.

Abstract

Filaments/prominences are cold plasma (~ 10000 K) embedded in the solar corona, two orders of magnitude hotter. Filament plasma is structured by the magnetic field in thin elongated threads. Counter-streaming flows have been observed. The aim of this paper is to characterize these flows. For that, we use high spatial resolution observations of spectral data obtained with THEMIS in H-alpha and with IRIS in Mg II k lines on 29 September 2023. We best detect counter-streaming flows in both the blue and red wings of these spectral lines. They are forming long Doppler shifted strands slightly inclined on the filament axis. The blue/red shift alternates across the strands at the arc second scale. H-alpha spectral profiles with large widths are interpreted as formed by multi-strands with opposite velocity directions. The absorption in the core of Mg II k line is also broader than in the chromosphere. This corresponds also to counter-streaming velocities. We derive that a fraction of the filament plasma is moving at supersonic speed (of the order of 20 km/s ) with the assumption that the filament is optically thick. We conclude that the counter-directed Doppler shifts might not be magnetic field aligned flows but rather correspond to kink transverse oscillations of the magnetic field with independent motions in nearby strands.