Granulation and the NLTE Formation Of K I 769.9
The diagnostic use of solar spectral lines often requires NLTE analysis of their formation. Such NLTE modeling is usually done for static model atmospheres. When dynamical structures such as the granulation or the 5-minute oscillation are studied, it is convenient to assume that the velocities and the temperature and density perturbations do not affect the size of the departures from LTE, so that the atomic level populations may be obtained directly by applying the static departure coefficients to the dynamic LTE populations. This approach has been used by Keil and Canfield (1978) for selected Fe I lines, and by Severino et al. (1986) for the K I 769.9 nm resonance line. We discuss this assumption here for the latter line. It is of interest because the K I resonance line attracts much attention as a helioseismological diagnostic.
We have used an atomic model for K I similar to the one specified by Severino et al. (1986) together with the Carlsson (1986) radiative transfer code to study the effects of granular perturbations on the K I departure coefficients.
Previous discussions of the granular influence on NLTE departures have typically addressed metal lines such as Fe I lines. For Fe I it is clear that overionization due to strong ultraviolet radiation fields is the main NLTE mechanism to be taken into account (Lites 1972, see also Rutten 1988). Nordlund (1984) has shown that the horizontal fluctuations related to granulation alter the NLTE iron ionization equilibrium drastically, and that the non-local nature of the ionization equilibrium has to be taken into account for realistic modeling of photospheric iron lines.
We find that ultraviolet overionization is also important in the case of potassium but less than for iron. The ionzation cross sections from the K I s-states are unusually small due to spin-orbit interaction (see Aymar et al. 1976, Sandner et al. 1981), and there is an effective compensation of the ultraviolet overionization for such low-energy bound-free transitions because they occur in the red part of the spectrum, for which the Planck function exceeds the mean intensity in the upper photosphere. In the case of K I, the ultraviolet overionization is cancelled by this recombination, so that the ground state population is very close to LTE throughout the photosphere, and actually overpopulated in the deep photosphere. The populations of the higher levels are lower than the LTE values, however, due to photon losses in the lines. This is just the reverse of the behaviour of the optical Fe I lines, of which the source functions obey LTE while their opacities are below the LTE values.
KeywordsResonance Line Granular Temperature Departure Coefficient Photon Loss Ground State Population
- Carlsson, M., 1986, Uppsala Astronomical Observatory Report No. 33Google Scholar
- Lites, B.W.: 1972, Observations and Analysis of the Solar Neutral Iron Spectrum, NCAR Cooperative Thesis no. 28, University of Colorado, BoulderGoogle Scholar
- Nordlund, Å.: 1984, in S.L. Keil (Ed.), Small-Scale Dynamical Processes in Quiet Stellar Atmospheres, National Solar Observatory Summer Conference, Sacramento Peak Observatory, Sunspot, p. 181Google Scholar
- Rutten, R.J.: 1988, in R, Viotti, A. Vittone, M. Friedjung (Eds.), Physics of Formation of Fe II Lines Outside LTE, IAU Colloquium 94, p. 185Google Scholar