ENERGY-BALANCE IN THE SOLAR TRANSITION REGION .2. EFFECTS OF PRESSURE AND ENERGY INPUT ON HYDROSTATIC MODELS

In this paper we address the question of how energy is radiated by hydrogen lines and continua hydrostatic energy-balance models of the transition region between the solar chromosphere and corona. Our models assume that mechanical or magnetic energy is dissipated in the hot corona and is then transported toward the chromosphere down the steep temperature gradient of the transition region. This model can be applied to the lower portions of magnetic flux tubes in which the temperature varies from chromospheric to coronal values. and to either open field or loop models. The basic difference between our calculations and those of other authors is that our theoretical hydrostatic energy-balance models properly take into account the diffusion of hydrogen atoms and ions. Diffusion, which was introduced in a previous paper, arises naturally due to the concentration and temperature gradients and is characterized by the ambipolar diffusion velocity. The diffusion effects are used in both the statistical equilibrium and energy balance equations and, as shown earlier. give order of magnitude agreement between calculations and the observations of the hydrogen Lyman continuum and Lyman lines in the average quiet Sun. The present paper describes various models constructed for the transition regions that correspond to different features of the solar atmosphere. Our calculations give the magnitude and distribution of energy radiated by the lower transition region after being transported down from the corona. Wc find that our models not only explain the average quiet Sun but also the entire range of variability of the Ly-alpha lines. We describe the relations between the downward energy flux, the pressure of the transition region, and the different hydrogen emissions. We show how the usual optically thin radiative loss curve can be modified to give agreement with our detailed calculations. and the extent to which this method can be used for estimating the actual radiative losses. Also, we show how the usual emission measure method can be modified to treat the Ly-alpha line emission, and we discuss the limitations of the emission measure approach.