Abstract
Data on the spectrum and center-to-limb variation of the solar Lyman continuum have been analyzed. They show: (a) The brightness temperature of the Lyman continuum is about 6500 K, but the kinetic temperature, as deduced from the slope of the continuum, lies between 8000 and 9000 K. The difference between the kinetic temperature and the brightness temperature requires that the source function be smaller than the Planck function by a factor of several hundred. (b) The Lyman continuum exhibits slight limb darkening longward of 825 Å, and slight limb brightening shortward of 750 Å. The crossover point varies from equator to pole and with solar activity. (c) The slope d ln I(λ)/dλ of the Lyman continuum decreases toward the limb, implying that the kinetic temperature increases outward in the region of Lyman continuum formation.
Using radiative transfer calculations for a plane-parallel atmosphere in hydrostatic equilibrium, we have derived a homogeneous model of the upper chromosphere that reproduces the main features of the observations. It is characterized by a temperature of 8300 K and a pressure of about 0.15 dyne/cm2 at τLyc = 1, and it has an abrupt temperature rise at a height of 1500 km above the limb. More precise agreement with the observations will require a detailed treatment of the inhomogeneous nature of the upper chromosphere.
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Noyes, R.W., Kalkofen, W. The solar Lyman continuum and the structure of the solar chromosphere. Sol Phys 15, 120–138 (1970). https://doi.org/10.1007/BF00149479
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DOI: https://doi.org/10.1007/BF00149479