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

Abstract

The conversion of fast waves to the Alfvén mode in a realistic sunspot atmosphere is studied through three-dimensional numerical simulations. An upward propagating fast acoustic wave is excited in the high-β 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°and low azimuths, while the Alfvén wave conversions peak at high inclinations and azimuths between 50°and 120°. Downward propagating Alfvén 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 Alfvén wave couples better with the downgoing fast magnetic wave which is reflected due to the gradients of the Alfvén speed. The simulations show that the Alfvén energy at the chromosphere is comparable to the acoustic energy of the slow mode, being even higher at high inclined magnetic fields. © © 2012. The American Astronomical Society. All rights reserved..