Transverse Wave Induced Kelvin-Helmholtz Rolls in Spicules

TL;DR

This paper investigates the role of transverse MHD waves and Kelvin-Helmholtz instabilities in spicule dynamics, combining observations and simulations to explain their structure, motion, and heating mechanisms.

Contribution

It introduces the concept of Transverse Wave Induced Kelvin-Helmholtz (TWIKH) rolls as a key factor in spicule coherence and dynamics, supported by observational and simulation evidence.

Findings

TWIKH rolls lead to coherent strand structures in observations.

Transverse dynamics cause rapid changes in Doppler velocities and strand visibility.

Temperature increases are mild, indicating additional heating processes.

Abstract

In addition to their jet-like dynamic behaviour, spicules usually exhibit strong transverse speeds, multi-stranded structure and heating from chromospheric to transition region temperatures. In this work we first analyse \textit{Hinode} \& \textit{IRIS} observations of spicules and find different behaviours in terms of their Doppler velocity evolution and collective motion of their sub-structure. Some have a Doppler shift sign change that is rather fixed along the spicule axis, and lack coherence in the oscillatory motion of strand-like structure, matching rotation models or long wavelength torsional Alfv\'en waves. Others exhibit a Doppler shift sign change at maximum displacement and coherent motion of their strands, suggesting a collective MHD wave. By comparing with an idealised 3-D MHD simulation combined with radiative transfer modelling, we analyse the role of transverse MHD waves and associated instabilities in spicule-like features. We find that Transverse Wave Induced Kelvin-Helmholtz (TWIKH) rolls lead to coherence of strand-like structure in imaging and spectral maps, as seen in some observations. The rapid transverse dynamics and the density and temperature gradients at the spicule boundary lead to ring-shaped \ion{Mg}{2} k and \ion{Ca}{2} H source functions in the transverse cross-section, potentially allowing IRIS to capture the KHI dynamics. Twists and currents propagate along the spicule at Alfv\'enic speeds, and the temperature variations within TWIKH rolls produce sudden appearance / disappearance of strands seen in Doppler velocity and in \ion{Ca}{2} H intensity. However, only a mild intensity increase in higher temperature lines is obtained, suggesting there is an additional heating mechanism at work in spicules.