Theoretical Departures from LTE for Neutral Sodium in the Solar Photosphere.

Abstract

Departures from LTE (local thermodynamic equilibrium) to be expected for neutral sodium in a solar model photosphere have been calculated theoretically. An itera- tive, self-consistent procedure is used to solve the steady-state and radiative transfer equations, so that the radiation field due to the atom is consistent with that in which the atom finds itself. Model sodium atoms of 2, 3, and 5 levels (3s, 3p, 4s, 3d, 4p) are studied to show the influence of the number of levels used. The model atmosphere is that used by the Paris Observatory group in their investigations of departures from LTE. The effect of different models of turbulence is studied. Numerical values of the nonequilibrium parameters bj are given throughout the photo sphere and low chromosphere. For a two-level atom, departures from LTE are found in the sense that the energy levels are overpopulated in the region T$ 0.001, underpopulated in the region 0.001 r 1, and have their equilibrium values for r >1 (T here being at 5000 A). The addition of more levels increases the overpopulation in the chromosphere but brings the populations very close to equilibrium in the photo- sphere. Reasons for this behavior are discussed. The effect of varying the collision cross sections and radiation fields is discussed. It is concluded that although large departures from LTE exist from neutral sodium in the chromosphere, theoretical calculations predict no significant departures in the solar photo sphere (below r 0.1, say). The source function for the Na D lines is discussed. It is pointed out that, the source function is collision- ally dominated, not photoionization dominated as had been expected on the basis of previous work by Jefferies and Thomas (Astrophys. J. 125, 260, 1957; Astrophys. J. 129, 401, 1959).