Friday, August 25, 2006

Convection in Astrophysics, Session A, Oral Contributions (Tue, Aug. 22)

S. Wedemeyer-Böhm: Dynamic Models of the Sun from the Convection Zone to the Chromosphere

This talk presented recent results from work with the CO5BOLD code (the code presented in the talk by Steffen). Classical studies of the solar chromosphere encounter the problem that UV and CO diagnostics give different results for the temperature as a function of depth.

The CO5BOLD code was used (including recent upgrades such as time-dependent chemistry) to simulate a box extending from -1400 to +1400 km vertically and 5000 km horizontally (where the photosphere is located at 0 km).

The resulting thermal structure shows that shockwave action dominates the upper layers. A fast evolving pattern of hot shock fronts is produced, with hot and cool temperatures next to each other. Thus, a temperature rise in the chromosphere can be "faked" by appropriate weighting, meaning e.g. that some of the diagnostics (spectral lines) might have a preference to form in hot regions.

The magnetic field was also studied, and the magnetic field strength pattern was found to evolve slowly in the lower layers (-1200 km, convection zone), fast in the upper layers (+1200, chromosphere) and with intermediate speed in the photosphere.

Further, the hydrogen ionisation was studied dynamically, and a different behaviour found depending on the equilibrium assumptions: In LTE, there are large gradients between high and low ionisation degrees, whereas with a non-equilibrium treatment there is much less variation of ionisation.

Finally, the results of a CO simulation where shown. In the upper layers (> 0 km), the relative abundance of CO is quite high, implying that a large fraction of carbon is bound in CO in chromosphere.

Future plans include more realistic models of the chromosphere, with time-dependent ionisation, non-LTE radiative transfer, larger models including a magnetic network, as well as detailed comparisons with obervations, e.g. ALMA.

F. Rincon: Anisotropy, Inhomogeneity And Inertial Range Scalings In Turbulent Convection

This talk was about inertial-range scaling laws in convection and the spectrum of turbulence in the solar photosphere.

It began with a reminder that in the Sun, turbulence is observed from above, which is a different perspective than in the laboratory.

Challenges of calculating turbulence in the Sun are that there is a boundary, which implies inhomogeneity, gravity is present as a force but also causes anisotropy, and there are plumes (/cells/eddies). So, the question was raised: what are the spectral scaling laws that we can observe?

Isotropic theories of turbulent convection have been around since 1941, when Kolmogorov derived an energy distribution E(k) proportional to k to the -5/3. In 1959, Bolgiano and Obhukov presented a more complex theory, including forcing, anisotropy and inhomogeneity. The Kolmogorov and the more complicated equations were now presented in the talk, but it is of course impossible to reproduce them here.

One point is that they involve an important length scale, the so-called Bolgiano length (LB), which is independent of the Nusselt, Rayleigh and Prandtl numbers. Unfortunately I do not recall how it is defined and what its meaning is, only that it defines a so-called "injection range": 1000 km < LB < 10000 km, which is confirmed by spectral budgets in polytropic convection.

The main conclusions were that photospheric turbulence is anisotropic and inhomogeneous and the anisotropy and forcing happens at an observable scale. Applications of this theory are presented in Yousef, Rincon and Shekochihin (2006). For further reading see Rincon (2006, J. Fluid Mech. 563, 43) and Rincon et al. (2005, A&A 430, L57).


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