SPECTRAL-LINE RADIATION FROM SOLAR SMALL-SCALE MAGNETIC-FLUX TUBES

We consider spectral line radiation from small-scale magnetic model fl ux tubes in the solar atmosphere. The structure of the tube is determi ned from the magnetostatic equations in the thin flux tube approximati on. We assume that the tube is in energy equilibrium and pressure bala nce 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 tu be models are parameterized by the plasma beta(0) (the ratio of gas th e pressure to the magnetic pressure), the convective efficiency parame ter alpha, and the radius R(0) at height z = 0 (tau(5000) =1) in the q uiet sun. The Stokes I and V profiles emerging from the models and ave raged over areas that include the neighbourhood of the flux tube are c alculated for various spectral lines with different sensitivity for ma gnetic field strength and temperature. The profiles are compared with high spatial resolution observations of plages near disc centre that h ave been obtained with the Gregory Coude Telescope at the Observatorio del Teide/Tenerife. The information contained in both I and V profile s is found to be very useful in constraining the theoretical models. T he best match of models with observations is achieved for values of be ta(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 requir ed. Pure velocity (microturbulent) broadening compatible with observat ions of strongly split lines gives too much broadening for weakly spli t lines. A broadening that is proportional to the Lande factor, i.e., magnetic broadening, appears to be more appropriate. This suggests dyn amic models with temporary enhancement of the magnetic field strength. The continuum intensity of our models is higher and the absorption an d V amplitude in the Fe II 6149 Angstrom line are stronger than observ ed. An improvement in the match between model predictions and observat ions is likely to come from models in which the ambient gas has a lowe r temperature as well as a lower temperature gradient than are found i n the quiet, field-free sun. Such models are currently under developme nt for cylindrical flux tubes.