Problems in Observation and Identification of Torsional Waves in the Lower Solar Atmosphere

TL;DR

This study critically examines the reliability of using spectral line width variations to detect torsional Alfvén waves in the lower solar atmosphere, highlighting limitations and proposing cautious interpretation of such observations.

Contribution

The paper provides an extensive observational analysis revealing that spectral line width variations are not exclusively indicative of torsional Alfvén waves, challenging the validity of this detection method.

Findings

Detected intensity oscillations at similar frequencies as line width variations.

Observed no center-to-limb dependency in the data.

Line profile broadening alone is insufficient for unambiguous wave identification.

Abstract

Registering periodic variations of spectral line widths serves as the main method for observing torsional Alfv\'{e}n waves. Theoretically, the method seems valid, yet it entails a number of caveats when applied to data. For instance, the amplitudes of these observations should vary with changes of the location on the disk, and they should be associated with no intensity oscillations. We analyze extensive observational material of periodic non-thermal variations of line widths in coronal holes and facular regions in a number of spectral lines: H{\alpha}, He I 10830 {\AA}, Ca II 8542 {\AA}, Ba II 4554 {\AA}. In most cases, we detected associated intensity oscillations at similar frequencies. Besides, we observed no centre-to-limb dependency. This calls for a discussion on the practical validity of the method and on the alternative explanations for the nature of non-thermal variations of spectral line widths. Based on our observations, we consider registering line profile broadening a necessary but not sufficient means for unambiguous identification of torsional Alfv\'{e}n waves in the lower solar atmosphere.