Realistic MHD Simulations of Solar Convection and Oscillations in Magnetic Regions: Mode Excitation and Effects of Acoustic Halos

We have used a 3D, compressible, non-linear radiative magnetohydrodynamics code developed at the NASA Ames Research Center to model solar convection and oscillations in magnetic regions. This code takes into account several physical phenomena: compressible fluid flow in a highly stratified medium, sub-grid scale turbulence models, radiative energy transfer, and a real-gas equation of state. We have studied the influence of the magnetic field of various strength on the convective cells and on the excitation mechanisms of the acoustic oscillations by calculating spectral properties of the convective motions and oscillations. The results reveal substantial changes of the granulation structure with increased magnetic field, and a frequency-dependent reduction in the oscillation power in a good agreement with solar observations. The simulations provide a solution to the long-standing problem of enhanced high-frequency acoustic emission at the boundaries of active region ("acoustic halos"), suggesting that this phenomenon is caused by the changes of the spatial-temporal spectrum of the turbulent convection in magnetic field, resulting in turbulent motions of smaller scales and higher frequencies than in quiet Sun regions. These simulation results may have also important implications for understanding high-frequency oscillations of magnetic stars.