Probing downflows in solar magnetic elements : the Fe ii test

In an attempt to assess the physical realism of the plage flux tube model derived by Bellot Rubio, Ruiz Cobo & Collados (1997, 1998) we extend its predictions to the Stokes V zero-crossing shifts of a number of Fe ii lines. For this analysis, accurate Fe ii central wavelengths are required. We have devised a procedure for bringing the available Fe ii laboratory wavelengths to the system of accurate Fe i wavelengths of Nave et al. (1994). It is shown that, relative to this system, the Fe ii system of Kurucz (1981) is shifted by 6.4 mÅ towards longer wavelengths. Some lines, however, are displaced by more than 10 mÅ. Corrected central wavelengths have been used to extract the observed Stokes V zero-crossing shifts of 16 Fe ii lines. Comparison with the values resulting from the model of Bellot Rubio et al. (1997, 1998) suggests that the velocity gradients derived by these authors are esentially correct.