Acoustic wave propagation in the solar subphotosphere with localised magnetic field concentration: effect of magnetic tension 

Sergiy Shelyag Submitted: 20090123 06:50
Aims. In this paper we analyse numerically the propagation and dispersion of acoustic waves in the solarlike subphotosphere with localised nonuniform magnetic field concentrations, mimicking sunspots with various representative magnetic field configurations.
Methods. Numerical simulations of wave propagation through the solar subphotosphere with a localised magnetic field concentration are carried out using SAC, which solves the MHD equations for gravitationally stratified plasma. The initial equilibrium density and pressure stratifications are derived from a standard solar model. Acoustic waves are generated by a source located at the height approximately corresponding to the visible surface of the Sun. We analyse the response of vertical velocity to changes in the interior due to magnetic field at the level corresponding to the visible solar surface, by the means of local timedistance helioseismology.
Results. The results of numerical simulations of acoustic wave propagation and dispersion in the solar subphotosphere with localised magnetic field concentrations of various types are presented. Timedistance diagrams of the vertical velocity perturbation at the level corresponding to the visible solar surface show that the magnetic field perturbs and scatters acoustic waves and absorbs the acoustic power of the wave packet. For the weakly magnetised case the effect of magnetic field is mainly thermodynamic, since the magnetic field changes the temperature stratification. However, we observe the signature of slow magnetoacoustic mode, propagating downwards, for the strong magnetic field cases.
Authors: S. Shelyag, S. Zharkov, V. Fedun, R. Erdelyi, M.J. Thompson
Projects: None

Publication Status: submitted
Last Modified: 20090123 09:36



Forward modelling of subphotospheric flows for timedistance helioseismology 

Sergiy Shelyag Submitted: 20070306 04:39
Results of forward modelling of acoustic wave propagation in a realistic solar subphotosphere with two cases of steady horizontal flows are presented and analysed by the means of local helioseismology. The simulations are based on fully compressible ideal hydrodynamical modelling in a Cartesian grid. The initial model is characterised by solar density and pressure stratifications taken from the standard Model S and is adjusted in order to suppress convective instability. Acoustic waves are excited by a nonharmonic source located below the depth corresponding to the visible surface of the Sun. Numerical experiments with coherent horizontal flows of linear and Gaussian dependences of flow speed on depth are carried out. These flow fields may mimic horizontal motions of plasma surrounding a sunspot, differential rotation or meridional circulation. An inversion of the velocity profiles from the simulated travel time differences is carried out. The inversion is based on the ray approximation. The results of inversion are then compared with the original velocity profiles. The influence of steady flow on the propagation of sound waves through the solar interior is analysed. Further, we propose a method of obtaining the traveltime differences for the waves propagating in subphotospheric solar regions with horizontal flows. The method employs directly the difference between traveltime diagrams of waves propagating with and against the background flow. The analysis shows that the flow speed profiles obtained from inversion based on the ray approximation differ from the original ones. The difference between them is caused by the fact that the wave packets propagate along the ray bundle, which has a finite extent, and thus reach deeper regions of the subphotosphere in comparison with ray theory.
Authors: S. Shelyag, R. Erdelyi, M. J. Thompson
Projects: None

Publication Status: A&A (submitted)
Last Modified: 20070306 13:53



Forward Modeling of Acoustic Wave Propagation in the Quiet Solar Subphotosphere 

Sergiy Shelyag Submitted: 20070305 08:33
The results of numerical simulations of acoustic wave propagation and dispersion in the nonmagnetic solar subphotosphere are presented. Initial equilibrium density and pressure stratifications are taken from a standard solar model but modified to suppress convective instabilities in fully compressible twodimensional ideal hydrodynamical modeling. Acoustic waves are generated by sources located below the height corresponding to the visible solar surface. The dynamic response of the solar interior to two acoustic source types, namely a harmonic source and one representing downwardpropagating photospheric plumes, is studied. A large number of randomly distributed localized cooling sources with random amplitudes is also introduced. The methods used to analyze the simulation data are similar to ones used in observational studies in local helioseismology. Timedistance diagrams of the pressure and vertical velocity perturbations at the level corresponding to the solar surface show the appearance of wave packets propagating with different speeds, which are reflected at different depths beneath the subphotosphere. The (ω, kh) power spectra, derived from the vertical velocity data, show the existence of g, f, and pmodes; pmode ridges are identifiable up to high radial orders of n~11; gmodes appear in the simulations, unlike in the real Sun, where they cannot propagate in the convectively unstable solar subphotosphere. Crosscorrelation analysis of vertical velocity perturbations shows a good correspondence with the observed timedistance helioseismic data for quiet Sun. Thus, the ability of the implemented approach of forward modeling to investigate propagation of acoustic, internal, and surface gravity waves in a realistic solar interior model is shown.
Authors: S. Shelyag , R. Erdelyi , and M. J. Thompson
Projects: None

Publication Status: ApJ, published
Last Modified: 20070306 14:00




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