Magnetoacoustic Waves in a Stratified Atmosphere with a Magnetic Null Point

TL;DR

This study uses nonlinear MHD simulations to analyze how magnetoacoustic waves propagate in a stratified atmosphere with a magnetic null point, revealing significant mode conversion, energy concentration, and dependence on wave parameters.

Contribution

It provides the first detailed analysis of wave behavior and energy conversion at a magnetic null point in a stratified atmosphere using nonlinear simulations.

Findings

Approximately 15.5% of wave energy converges on the null.

70% of incident energy on the null converts to slow modes.

Refracting waves and current buildup depend on wavevector and initial position.

Abstract

We perform nonlinear MHD simulations to study the propagation of magnetoacoustic waves from the photosphere to the low corona. We focus on a 2D system with a gravitationally stratified atmosphere and three photospheric concentrations of magnetic flux that give rise to a magnetic null point with a magnetic dome topology. We find that a single wavepacket introduced at the lower boundary splits into multiple secondary wavepackets. A portion of the packet refracts towards the null due to the varying Alfv\'en speed. Waves incident on the equipartition contour surrounding the null, where the sound and Alfv\'en speeds coincide, partially transmit, reflect, and mode convert between branches of the local dispersion relation. Approximately $15.5\%$ of the wavepacket's initial energy ($E_{input}$) converges on the null, mostly as a fast magnetoacoustic wave. Conversion is very efficient: $70\%$ of the energy incident on the null is converted to slow modes propagating away from the null, $7\%$ leaves as a fast wave, and the remaining $23\%$ (0.036$E_{input}$) is locally dissipated. The acoustic energy leaving the null is strongly concentrated along field lines near each of the null's four separatrices. The portion of the wavepacket that refracts towards the null, and the amount of current accumulation, depends on the vertical and horizontal wavenumbers and the centroid position of the wavepacket as it crosses the photosphere. Regions that refract towards or away from the null do not simply coincide with regions of open versus closed magnetic field or the local field orientation. We also modeled wavepacket propagation using a WKB method and found that it agrees qualitatively, though not quantitatively, with the results of the numerical simulation.