Spectral line radiation from solar small-scale magnetic flux tubes

We consider spectral line radiation from small-scale magnetic model flux tubes in the solar atmosphere. The structure of the tube is determined from the magnetostatic equations in the thin flux tube approximation. We assume that the tube is in energy equilibrium and pressure balance with the ambient medium. For the latter, we construct a quiet sun model with an artificial heating term in order to reproduce the VAL C model, treating the medium as a plane-parallel atmosphere. The flux tube models are parameterized by the plasma beta(0) (the ratio of gas the pressure to the magnetic pressure), the convective efficiency parameter alpha, and the radius R(0) at height z = 0 (tau(5000) =1) in the quiet sun. The Stokes I and V profiles emerging from the models and averaged over areas that include the neighbourhood of the flux tube are calculated for various spectral lines with different sensitivity for magnetic field strength and temperature. The profiles are compared with high spatial resolution observations of plages near disc centre that have been obtained with the Gregory Coude Telescope at the Observatorio del Teide/Tenerife. The information contained in both I and V profiles is found to be very useful in constraining the theoretical models. The best match of models with observations is achieved for values of beta(0) between 0.3 and 0.5. For a sufficiently wide separation of the V extrema of the strongly split lines, a broadening mechanism is required. Pure velocity (microturbulent) broadening compatible with observations of strongly split lines gives too much broadening for weakly split lines. A broadening that is proportional to the Lande factor, i.e., magnetic broadening, appears to be more appropriate. This suggests dynamic models with temporary enhancement of the magnetic field strength. The continuum intensity of our models is higher and the absorption and V amplitude in the Fe II 6149 Angstrom line are stronger than observed. An improvement in the match between model predictions and observations is likely to come from models in which the ambient gas has a lower temperature as well as a lower temperature gradient than are found in the quiet, field-free sun. Such models are currently under development for cylindrical flux tubes.