Numerical simulation of excitation and propagation of helioseismic MHD waves: Effects of inclined magnetic field

Abstract

Investigation of propagation, conversion, and scattering of MHD waves in the Sun is very important for understanding the mechanisms of observed oscillations and waves in sunspots and active regions. We have developed 3D linear MHD numerical model to investigate influence of the magnetic field on excitation and properties of the MHD waves. The results show that the magnetic field can substantially change the properties of the surface gravity waves (f-mode), but their influence on the acoustic-type waves (p-modes) is rather moderate. Comparison our simulations with the time-distance helioseismology results from SOHO/MDI shows that the travel time variations caused by the inclined magnetic field do not exceed 25% of the observed amplitude even for strong fields of 1400-1900 G. This can be an indication that other effects (e.g. background flows and non-uniform distribution of magnetic field) can contribute to the observed travel time variations. The travel time variations caused by the wave interaction with magnetic field are in phase with the observations for strong fields of 1400-1900 G if Doppler velocities are taken at the height of 300 km above the photosphere where plasma parameter beta<<1. The simulations show that the travel times only weakly depend on the height of velocity observation. For the photospheric level the travel times are systematically smaller on approximately 0.12 min then for the hight of 300 km above the photosphere for all studied ranges of the magnetic field strength and inclination angles. The numerical MHD wave modeling and new data from the HMI instrument of the Solar Dynamics Observatory will substantially advance our knowledge of the wave interaction with strong magnetic fields on the Sun and improve the local helioseismology diagnostics.

24 pages, 10 figures, submitted to ApJ

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