Numerical simulations of conversion to Alfvén waves in solar active regions

We study the coupling of magneto-acoustic waves to Alvén waves using 2.5D numerical simulations. In our experiment, a fast magnetoacoustic wave of a given frequency and wavenumber is generated below the surface. The magnetic field in the domain is assumed homogeneous and inclined. The efficiency of the conversion to Alfvén waves near the layer of equal acoustic and Alfven speeds is measured calculating their energy flux. The particular amplitude and phase relations between the oscillations of magnetic field and velocity help us to demonstrate that the waves produced after the transformation and reaching upper atmosphere are indeed Alfvén waves. We find that the conversion from fast magneto-acoustic waves to Alfvén waves is particularly important for the inclination θ and azimuth phi angles of the magnetic field between 55 and 65 degrees, with the maximum shifted to larger inclinations for lower frequency waves. The maximum Alfvén flux transmitted to the upper atmosphere is about 2-3 times lower than the corresponding acoustic flux.