Convection
Presiding: N Hurlburt, Lockheed Martin Advanced Technology Center; S L Keil, National Solar Observatory
SP11C-01 08:30h
Large-Scale Solar Photosphere Simulations
We have developed a 3D, compressible radiative-hydrodynamics code for simulating the upper solar photosphere and lower atmosphere, from depths of a few 10's of megameters below to an altitude of a few hundred kilometers above the visible surface. Real gas opacities and equation of state are used, and the radiation is captured through full 3D solution of the radiative transfer equation in opacity bands. The code is fully parallelized using the Message-Passing Interface (MPI) standard, allowing execution on both distributed and shared-memory architectures. We have benchmarked the code on the NASA Columbia system at Ames (an SGI Altix computer), obtaining near-ideal, linear scaling for a 500× 500× 500 node spatial mesh using from 1 to 500 processors. This scaling behavior gives us confidence that the code can be used to run large meshes effectively on large numbers of processors. We will show preliminary results from a simulation of a box 20 Mm × 20 Mm × 18 Mm at a resolution (Δ x) of approximately 40km, with a 5123 grid. Granular and intergranular lane structure and dynamics, turbulent statistics, thermal, and radiative properties will be described and compared to observations. Testing of subgrid-scale turbulence models will be discussed.
SP11C-02 08:45h
Numerical Simulations of Bipolar Magnetic Field Decay in Turbulent Convection
We present numerical simulations of compressible magnetoconvection in spherical segments, seeking to examine the decay of active region magnetic fields on the sun. It is surprising that after their emergence, active regions are observed to persist in relative stasis for long periods of time (weeks to months) before suddenly disintegrating. We perform a series of calculations to investigate this process, in which we drive turbulent convection (Rayleigh numbers of order 107) within two- and three-dimensional spherical segments, and measure the decay rates of the embedded bipolar magnetic fields.
SP11C-03 09:00h
Solar Supergranulation as Propagating Waves
It has been observed that the supergranulation pattern on the surface of the Sun appears to rotate faster than the photospheric plasma and magnetic features. It is postulated that this could be due to instabilities in the subsurface convective shear layer. This behaviour is modelled, starting with a linearized system of differential equations describing a convectively unstable region containing a horizontal shear flow. The system is solved numerically, using parameters drawn from the solar model and helioseismic inversions and assuming linear and non-linear shear flow profiles, for a range of wavenumbers. The phase speeds of the resulting wave solutions are found to be greater than the surface speed, possibly explaining the observed behaviour.
SP11C-04 09:15h
Depth Dependence of Solar Supergranular Flow
Helioseismic inversions for subsurface supergranular motion have been performed using spectral-domain covariance estimates of the surface wave field as input data. The surface wave measurements used in this study were obtained from a 34-hr sequence of SOHO/MDI high-resolution Doppler images covering an approximately 210 Mm x 210 Mm corotating patch of the photosphere near disk center. The utilized signal, derived by filtering the Doppler sequence, is dominated by p- and f-modes in the frequency range 2.5 - 5.5 mHz and the range 600 - 1800 of angular degree. The helioseismic forward model used for the inversions is based on a wave equation which includes a stochastic driving term and a damping term. The effect of subsurface flows on wave propagation is treated in the single-scattering Born approximation. Several approaches to inverting helioseismic covariance data were used, including the method of optimally-localized averages. The inversions provide evidence for supergranular-scale motion to a detection depth of 6 Mm beneath the photosphere. The measured correlation coefficient between the surface motion and the motion at depth is observed to decrease with depth, but, contrary to some previous findings, does not change sign over the detectable depth range. The author is indebted to Yuhong Fan and Aaron Birch for extensive discussions about the theoretical aspects of wave propagation in flows and to the former for vital solar model outputs. The study was supported by NSF grant ATM-0223127.
SP11C-05 09:30h
Supergranulation Scale Solar Convection Simulations
Supergranulation scale (50 Mm wide by 20 Mm deep) simulations of solar convection are being relaxed thermally and dynamically. The initial state was made by duplicating a periodic smaller simulation of 24 Mm wide by 9 Mm deep and extending it in depth assuming constant entropy upflows and extrapolating the downflows. Relaxation is rapid near the surface, but very slow at large depths and large horizontal scales. Initial results are reported. These simulations will help separate the role of the second helium ionization zone from the effect of the increasing scale height with depth. This large size is also necessary for analyzing local helioseismic inversion techniques. Coriolis forces becomes significant on these spatio-temporal scales and we have added f-plane rotation to investigate the nature of the surface shear layer. Eventually, magnetic fields will be added to study the development and maintenance of the magnetic network.
http://www.pa.msu.edu/~steinr/research.html