Three-dimensional numerical simulations of fast-to-Alfven conversion in sunspots

TL;DR

This study uses 3D numerical simulations to analyze how fast waves convert into Alfven and slow modes in sunspot atmospheres, revealing specific inclination and azimuth conditions for efficient mode conversion and significant Alfven energy in the chromosphere.

Contribution

It provides the first detailed 3D simulation analysis of fast-to-Alfven wave conversion in realistic sunspot models, highlighting the conditions for maximum efficiency and energy distribution.

Findings

Maximum slow mode conversion at 25° inclination and low azimuths.

Peak Alfven conversion occurs at high inclinations and azimuths between 50° and 120°.

Alfven energy in the chromosphere can surpass acoustic energy, especially in highly inclined magnetic fields.

Abstract

The conversion of fast waves to the Alfven mode in a realistic sunspot atmosphere is studied through three-dimensional numerical simulations. An upward propagating fast acoustic wave is excited in the high-beta region of the model. The new wave modes generated at the conversion layer are analyzed from the projections of the velocity and magnetic field in their characteristic directions, and the computation of their wave energy and fluxes. The analysis reveals that the maximum efficiency of the conversion to the slow mode is obtained for inclinations of 25 degrees and low azimuths, while the Alfven wave conversions peaks at high inclinations and azimuths between 50 and 120 degrees. Downward propagating Alfven waves appear at the regions of the sunspot where the orientation of the magnetic field is in the direction opposite to the wave propagation, since at these locations the Alfven wave couples better with the downgoing fast magnetic wave which are reflected due to the gradients of the Alfven speed. The simulations shows that the Alfven energy at the chromosphere is comparable to the acoustic energy of the slow mode, being even higher at high inclined magnetic fields.