Three-Dimensional Modeling of Solar Convection and Atmosphere Dynamics
Abstract
Results from non-magnetic numerical 3D simulations of the solar atmosphere and the convection zone below are presented. I find that transient acoustic wave trains in the atmosphere are excited by smaller granular cells that undergo a rapid collapse in the sense that upflow is reversed to downflow on a timescale shorter than the atmospheric acoustic cutoff period. An extended downdraft between larger cells is formed at the site of the collapse. The main wave excitation sources are found at the top of the convective layer (the cooling layer) and immediately above in the convective overshoot layer. The excitation lasts for 2-3 minutes as the upflow in the granule reverses to downflow. The following vertical wave components are long wavelength, gravity modified acoustic waves with periods close to the 3 minute eigenoscillation of the solar atmosphere. These waves shock at chromospheric layers. The oscillation is initially horizontally localized with a size of about 1 Mm. The wave amplitude decay in time as energy is transported horizontally and vertically away from the site of the event. This is the only convectively generated wave source I find that shows a clear correlation to large scale atmospheric wave motions. Observed darkening of intergranular lanes and the associated photospheric wave motions, the so called ``acoustic events'' could be explained by this purely hydrodynamical process. Furthermore, the observed ``internetwork bright grains'' in the CaII H and K line cores and associated shock wave trains in the chromosphere can also be linked to this wave train. The simulation is an extended Nordlund and Stein model of solar convection, so as to include the chromosphere. A large part of the work has been spent on developing a new method for calculating radiative flux divergence in the simulation, in which I treat photon scattering in atmospheric layers. The previous NS-model used the Planck function as source function, while the improved method includes a scattering term, which introduces a global radiation problem, requiring an iterative solution at each timestep in the simulation. By assuming opacity in LTE and coherent isotropic scattering, I calculate group mean opacity coefficients to be used in a group mean source function. This source function contains an approximate scattering term and an exact contribution from thermal emissivity. The resulting three dimensional scattering problems are solved by iteration using a new method based on a one-ray approximation in the angle integral for the mean intensity. The equations to be iterated are tri-diagonal matrix equations which require a minimum of computer time. I have compared exact wavelength integrated monochromatic solutions with the corresponding approximate group mean solutions for solar conditions. I find that the total flux divergence obtained from groups deviates with less than 10 % from the exact solution. Flux divergence in individual groups can deviate with typically 30 % in atmospheric layers. When using these groups, the CPU time is reduced by a factor of about 100 in a test case for solar conditions. The main difference for the atmospheric dynamics between LTE and scattering solutions, is that the wave amplitudes in the chromosphere are larger with scattering. This is because the radiative damping is smaller than in LTE for a given wave amplitude. The convection dynamics and therefore the convective wave sources are almost unaltered, since the radiative cooling that drives the convection is mainly unchanged. Collapsing granules is therefore also expected in LTE, but the chromospheric response would be of smaller amplitude, and the shock train would form at higher altitudes.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1998
- Bibcode:
- 1998PhDT........34S