Magnetic shocks and substructures excited by torsional Alfv\'en wave interactions in merging expanding flux tubes

TL;DR

This study uses numerical simulations to show how vortex-driven wave interactions in merging solar flux tubes generate shocks and magnetic substructures, significantly contributing to energy transfer and heating from the lower atmosphere to the corona.

Contribution

It introduces a new mechanism where vortex interactions in merging flux tubes produce shocks and complex magnetic structures, facilitating energy transfer in the solar atmosphere.

Findings

Vortex motions cause the flux tubes to fragment into multi-threaded structures.

Wave interactions generate shocks exceeding Mach 1, propagating at ~50 km/s.

Heating of plasma increases by over an order of magnitude, reaching ~60,000 K.

Abstract

Vortex motions are frequently observed on the solar photosphere. These motions may play a key role in the transport of energy and momentum from the lower atmosphere into the upper solar atmosphere, contributing to coronal heating. The lower solar atmosphere also consists of complex networks of flux tubes that expand and merge throughout the chromosphere and upper atmosphere. We perform numerical simulations to investigate the behaviour of vortex driven waves propagating in a pair of such flux tubes in a non-force-free equilibrium with a realistically modelled solar atmosphere. The two flux tubes are independently perturbed at their footpoints by counter-rotating vortex motions. When the flux tubes merge, the vortex motions interact both linearly and nonlinearly. The linear interactions generate many small-scale transient magnetic substructures due to the magnetic stress imposed by the vortex motions. Thus, an initially monolithic tube is separated into a complex multi-threaded tube due to the photospheric vortex motions. The wave interactions also drive a superposition that increases in amplitude until it exceeds the local Mach number and produces shocks that propagate upwards with speeds of approximately $ 50$ km s$^{-1}$. The shocks act as conduits transporting momentum and energy upwards, and heating the local plasma by more than an order of magnitude, with peak temperature approximately $60,000$ K. Therefore, we present a new mechanism for the generation of magnetic waveguides from the lower solar atmosphere to the solar corona. This wave guide appears as the result of interacting perturbations in neighbouring flux tubes. Thus, the interactions of photospheric vortex motions is a potentially significant mechanism for energy transfer from the lower to upper solar atmosphere.