Probing downflows in solar magnetic elements: the Fe II test

In an attempt to assess the physical realism of the plage flux tube model d erived by Bellot Rubio, Ruiz Cobo & Collados (1997, 1998) we extend its pre dictions to the Stokes V zero-crossing shifts of a number of Fe II lines. F or 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 sh own that, relative to this system, the Fe II system of Kurucz (1981) is shi fted by 6.4 m Angstrom towards longer wavelengths. Some lines, however, are displaced by more than 10 m Angstrom. Corrected central wavelengths have b een 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 a uthors are esentially correct.