A STOCHASTIC-MODEL OF THE SOLAR ATMOSPHERE

We present a model for the lower solar atmosphere based on continuum o bservations of the Sun spanning the 2-1200 mu m wavelength range. We h ave shown that the data, in particular the center-to-limb brightness p rofiles at 50-350 mu m, cannot be accounted for by any model which is plane-parallel and homogeneous in the height range in which this radia tion is formed. We accordingly set out to develop a two-component mode l as the natural generalization. Making use of a theory for radiation transfer in a stochastic multi-component atmosphere, we find that one can indeed obtain an inhomogeneous model which satisfies center-to-lim b data over the 2-1200 mu m range. This composite model is made up of hot ''flux tubes'' randomly embedded in a cool medium, the flux tubes expanding to occupy an increasing proportion of the atmosphere as we m ove up in height. The cool ambient component shows a monotonic decreas e in temperature in the range defined by the data. The temperature in the hot component is constant at about 6500 K up to about 400 km and i ncreases monotonically above that height. The center-to-limb observati ons demand that the gas in the interiors of the flux tubes be recessed downward with respect to a hydrostatic equilibrium distribution of de nsity. This appears to constitute a chromospheric Wilson depression co nsistent with a magnetic field of about 120 G in the flux-tube interio r at a height of about 600 km. The new model is shown to be consistent with other spectral measurements independent of those used to define it. It gives a very good fit to the 0.5 mu m continuum intensities acr oss the disk, and provides an excellent accounting for the disk-center brightness temperature in the center of the 3-2 R14 CO line at 4.667 mu m. A boundary temperature of less than about 3000 K in the cold com ponent is suggested from the limb-darkening data available for this li ne. In an appendix we mention a procedure for an analogous study based on the intensities of multiplet lines, which may hold promise for mod eling over a wider range of heights that can be spanned by the IR data .