Generating transverse loop oscillations through a steady-flow driver

TL;DR

This study demonstrates through 3D MHD simulations that steady background flows can self-sustain transverse oscillations in coronal loops, providing a potential explanation for observed decay-less oscillations without periodic drivers.

Contribution

It introduces a novel mechanism where a steady flow drives self-sustained transverse oscillations in coronal loops, expanding understanding beyond periodic footpoint excitation models.

Findings

Fundamental kink mode oscillations are excited by steady flows.

A second harmonic with a realistic frequency ratio is observed.

Vortex shedding occurs, indicating complex flow-oscillation interactions.

Abstract

In recent years, the decay-less regime of standing transverse oscillations in coronal loops has been the topic of many observational and numerical studies, focusing on their physical characteristics, as well as their importance for coronal seismology and wave heating. However, no definitive answer has yet been given on the driving mechanism behind these oscillations, with most studies focusing on the use of periodic footpoint drivers as a means to excite them. In this paper, our goal is to explore the concept of these standing waves being self-sustained oscillations, driven by a constant background flow. To that end, we use the PLUTO code, to perform $3$D magnetohydrodynamic simulations of a gravitationally stratified straight flux tube in a coronal environment, in the presence of a weak flow around the loop, perpendicular to its axis. Once this flow is firmly set up, a transverse oscillation is initiated, dominated by the fundamental kink mode of a standing wave, while the existence of a second harmonic is revealed, with a frequency ratio to the fundamental mode near the observed ones in decay-less oscillations. The presence of vortex shedding is also established in our simulations, which is connected to the "slippery" interaction between the oscillator and its surrounding plasma. We thus present a proof-of-concept of a self-oscillation in a coronal loop, and we propose it as a mechanism that could interpret the observed decay-less transverse oscillations of coronal loops.