Five-minutes oscillations is one of the basic properties of solar convection. Observations show mixture of a large number of acoustic wave fronts propagating from their sources. We investigate the process of acoustic waves excitation from the point of view of individual events, by using realistic 3D radiative hydrodynamic simulation of the quiet Sun. The results show that the excitation events are related to dynamics vortex tubes (or swirls) in the intergranular lanes. These whirlpool-like flows are characterized by very strong horizontal velocities (7 - 11 km/s) and downflows (~ 7 km/s), and are accompanied by strong decreases of the temperature, density and pressure at the surface and in a ~ 0.5-1 Mm deep layer below the surface. High-speed whirlpool flows can attract and capture other vortices. According to our simulation results, the processes of the vortex interaction, such as vortex annihilation, can cause the excitation of acoustic waves.
Oscillatory behavior is one of the basic properties of the solar surface. Stochastic wave excitation in the turbulent convective medium and frequentative interference of the wavefronts make the problem of solar oscillations very complicated. In observational data, individual wave excitation events are overlaped with other waves. Nevertheless, individual impulsive events of acoustic waves excitation have been detected by the Vacuum Tower Telescope (Sacramento Peak Observatory). According to the data analysis and the proposed numerical model, the source of solar acoustic waves with a typical period of 5 min is located in the turbulent convection layer ∼ 200 km below the solar surface (Goode et al. 1992). Such impulsive events usually occur in the intergranular lanes of granulation, and are associated with locally high cooling (Rimmele et al. 1995).
At present time there is no clear explanation of the mechanism of acoustic wave sources on the Sun. In the turbulence theory, the wave excitation process was initially studied by Lighthill (1952) for the waves generation in air flow. According to theory, the process of transformation of the flow kinetic energy to the acoustic energy can be caused by shorttime deformation of fluid elements by the influence of a shear flow (Lighthill 1954). The importance of turbulence for the wave excitation on the Sun was confirmed by numerical simulations. In particular, the numerical results of Stein & Nordlund (2001) supported the possibility of the near surface location of the acoustic sources. The simulations also supported the suggestion that the sources are located in the intergranular lanes or in their vicinity, where the wave excitation is related to occasional, strong pressure fluctuations associated with the initiation of dowdrafts, and, sometimes with the intergranular lane formation. However, numerical simulations cannot resolve turbulent scales in the solar conditions, therefore accurate sub-grid scale modeling of the turbulence is important. In particular, Jacoutot et al. (2008a) studied several turbulence models, and found that the best agreement of the synthetic oscillation power spectrum with observational data is given by the dynamic formulation of the Smagorinsky model (Germano et al. 1991;Moin et al. 1991).
The problem of resolving small-scale turbulent effects exists also in the observations. Recent observations were able to resolve relatively large, ∼ 3 Mm in diameter, vortex flows in the photosphere (Brandt et al. 1988). Previously, an evidence for vortex motions was found from an example of two bright points rotating around each other (Wang et al. 1995). Other observations of vortices showed connection of the vortex motions to strong downflows (Pötzi & Brandt 2005). Also, the observations were able to detect a number of vortical flows in a quiet Sun region, near the solar disk center, by tracing magnetic bright points (Bonet et al. 2008), which followed a logarithmic spiral trajectory around intergranular points and were engulfed by a downdraft. These whirlpools have the size ≤ 0.5 Mm, and their lifetime varies from 5 min to ≥ 20 min (Bonet et al. 2008(Bonet et al. , 2010)). The search and identification problem of vortices in solar data is mainly the problem of spatial resolution of the observations. The distribution of vortices shows a strong preference to concentration in regions of convective downflows, particulary, at the mesogranular scale (Pötzi & Brandt 2005).
In this paper, we present new results of 3D radiative hydrodynamics numerical simulations of a quiet Sun region, identify the process of excitation of acoustic waves, and study their propagation in the convective medium. Our results reveal an interesting connection between a process of interaction of vortex tubes, distributed in the near-surface layer of the Sun, and the generation of acoustic waves.
We used the 3D radiative MHD simulation code (“SolarBox”, developed by A. Wray). It is based on a Large Eddy Simulations (LES) formulation and includes various subgrid-scale turbulence models (Jacoutot et al. 2008a). This code takes into account several physical phenomena: compressible fluid flow in a highly stratified medium, three-dimensional multigroup radiative energy transfer between the fluid elements, a real-gas equation of state, ionization and excitation of all abundant species. The code has been carefully tested, and used for studying how various turbulence models affect the excitation of solar acoustic oscillations by turbulent convection in the upper convection zone (Jacoutot et al. 2008a,b). The code was verified by comparison with the code of Stein & Nordlund (2001), and has been used to study solar oscillations (Jacoutot et al. 2008a), magnetoconvection of sunspots (Kitiashvili et al. 2009), and processes of spontaneous formation of magnetic structures (Kitiashvili et al. 2010).
The simulation results are obtained for computational domains of 6.4×6.4×5.5 Mm 3 and 12 × 12 × 5.5
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