THE CHROMOSPHERIC TEMPERATURE RISE IN SOLAR MAGNETIC-FLUX TUBES

We set constraints on the location and steepness of the chromospheric temperature rise in solar magnetic flux tubes by synthesizing Stokes V profiles in non-local thermodynamic equilibrium (NLTE) of a set of ''clean'' Fe I and Fe ii lines of different strengths. We compute profiles for several flux-tube models of the magnetic features within solar network and plage. Comparison with observed V profiles indicates that both in network and plage the chromospheric temperature rise starts at larger optical depth within the flux tubes than in the surrounding non-magnetic atmosphere. The exact onset cannot be determined with the present set of lines because they cannot easily distinguish between the location and initial steepness of the temperature rise. Assuming a similar T(tau) gradient as in quiet-Sun models, the chromosphere sets in 200 - 300 km deeper in flux tubes than in the quiet Sun. The similarity between the plage and network results suggests that in the low chromosphere the heating per flux tube is almost independent of magnetic filling factor.