Identification of different types of kink modes in coronal loops: principles and application to TRACE results

TL;DR

This study investigates how different kink modes in coronal loops can be identified from imaging and Doppler data, revealing challenges and potential solutions for mode discrimination in solar observations.

Contribution

It introduces a geometrical model for kink modes and applies it to TRACE data, highlighting the difficulties in mode identification and the usefulness of Doppler signatures.

Findings

Mode identification is challenging due to viewing geometry.

Doppler signatures assist in distinguishing oscillation modes.

Some observed loops are misclassified as fundamental horizontal modes.

Abstract

We explore the possible observational signatures of different types of kink modes (horizontal and vertical oscillations in their fundamental mode and second harmonic) that may arise in coronal loops, with the aim of determining how well the individual modes can be uniquely identified from time series of images. A simple, purely geometrical model is constructed to describe the different types of kink-mode oscillations. These are then `observed' from a given direction. In particular, we employ the 3D geometrical parameters of 14 TRACE loops of transverse oscillations to try to identify the correct observed wave mode. We find that for many combinations of viewing and loop geometry it is not straightforward to distinguish between at least two types of kink modes just using time series of images. We also considered Doppler signatures and find that these can help obtain unique identifications of the oscillation modes when employed in combination with imaging. We then compare the modeled spatial signatures with the observations of 14 TRACE loops. We find that out of three oscillations previously identified as fundamental horizontal mode oscillations, two cases appear to be fundamental vertical mode oscillations (but possibly combined with the fundamental horizontal mode), and one case appears to be a combination of the fundamental vertical and horizontal modes, while in three cases it is not possible to clearly distinguish between the fundamental mode and the second-harmonic of the horizontal oscillation. In five other cases it is not possible to clearly distinguish between a fundamental horizontal mode and the second-harmonic of a vertical mode.