Polarization of decayless kink oscillations in a 3D MHD coronal loop model

TL;DR

Decayless kink oscillations in solar coronal loops have uncertain drivers, but polarization offers a diagnostic. Using a self-consistent 3D MHD loop-in-a-box modeled with MURaM and synthetic 171 Å EUV emission, the authors analyze polarization via velocity hodograms and phase relationships of loop threads. They find persistent, low-amplitude decayless kink waves with 40–60 s periods, indicating linear polarization with tilt relative to principal axes, and that these oscillations arise without an imposed driver, favoring self-sustained or quasi-steady drivers over stochastic sources. The results support polarization-based constraints on wave drivers and have implications for coronal heating by kink-mode waves.

Abstract

Decayless kink oscillations are frequently observed in solar coronal loops and are considered potential contributors to coronal heating. Despite the ubiquity of this wave phenomenon, its driving mechanism remains unclear. Studies to derive the polarization state of these oscillations, which would be a key to identifying the drivers, have been limited due to observational constraints. We analyze a 3D MHD simulation of coronal loops using the MURaM code. Synthetic extreme ultraviolet (EUV) emission maps, combined with velocity diagnostics, are used to identify and characterize transverse wave motions in the simulated loop structures. This is the first demonstration of decayless kink waves emerging self-consistently in a 3D MHD loop-in-a-box model. The simulation produces persistent, low-amplitude, decayless kink oscillations that closely match observed properties. These oscillations arise spontaneously, without any imposed periodic driver, and exhibit clear linear polarization with oscillation planes not aligned to the principal axes. The observed coherency of linear polarization with oscillation cycles favors a self-sustained or quasi-steady type wave driver over a stochastic or broadband source.

Figures (7)

• Figure 1: An overview of the $x$-$t$ maps generated from the synthetic 171 Å image sequence. Panel a1 illustrates the schematic of the simulation domain, where the orange hatched area outlines the volume used to generate the synthetic image of case 1, shown in panel a2. The white rectangular box in panel a2 indicates the location and extent of the artificial slit used to create the space-time ($x$-$t$) map displayed in panel a3. The two vertical lines in this $x$-$t$ map define the time window during which decayless oscillations are observed . Cases 2 and 3 are presented in panels b1 to b3 and c1 to c3, respectively.
• Figure 2: Signatures of decayless oscillations in emission $x$-$t$ maps and in the velocity components. Column-a presents signatures of oscillation-1 (from case-1). In this column, the top row displays a zoomed-in $x$-$t$ map from Fig. \ref{['fig:context_plot']}, while the next three rows illustrate the time evolution of the velocity components ${v_x}$, ${v_y}$ and ${v_z}$, respectively. The white horizontal lines indicate the $\delta$x extent of the 3D volume from which the velocity components were obtained. See Sect. \ref{['sec:decayless']} for more details. The other two columns (b and c) present information for oscillations 2 and 3 in the same format.
• Figure 3: Hodograms of the three decayless kink oscillations. In each instance, the colors in these hodograms represent the flow of time. An animated version is available https://drive.google.com/drive/folders/1ul9ZDnsz77p9y-cPO5IVPfcqABkThlZG?usp=sharing.
• Figure 4: An example of co-existing decaying and decayless kink oscillations in neighboring loops. Top row shows the synthetic 171 Å image with the white rectangular box outlining the position of the artificial slit used to generate the $x$-$t$ map shown in next panel. The slanted white and black dotted lines mark the $\delta$x values for the two selected oscillations. For more information on this, see Sect \ref{['sec:decayless']}. The middle row displays the ${v_x}$, ${v_y}$ and ${v_z}$ curves and the hodograms constructed for $\textit{oscillation-4.1}$. The same but for $\textit{oscillation-4.2}$ is shown in the bottom row panels.
• Figure 5: Original and detrended velocity curves for each oscillation. For a given oscillation, the original curves are shown in grey, while the detrended ${v_x}$, ${v_y}$, and ${v_z}$ curves are displayed with green, red, and blue lines, respectively.