  As a part of the long-term program at Kitt Peak National Observatory (KPNO),
the Mn I 539.4 nm line has been observed for nearly three solar cycles using
the McMath telescope and the 13.5 m spectrograph in double-pass mode. These
full-disk spectrophotometric observations revealed an unusually strong change
of this line's parameters over the solar cycle. Optical pumping by the Mg II k
line was originally proposed to explain these variations. More recent studies
have proposed that this is not required and that the magnetic variability might
explain it. Magnetic variability is also the mechanism that drives the changes
in total solar irradiance variations (TSI). With this work we investigate this
proposition quantitatively by using using the model SATIRE-S. We applied
exactly the same model atmospheres and value of the free parameter as were used
in previous solar irradiance reconstructions to now model the variation in the
Mn I 539.4 nm line profile and in neighboring Fe I lines. We compared the
results of the theoretical model with KPNO observations. Our result confirms
that optical pumping of the Mn I 539.4 nm line by Mg II k is not the main cause
of its solar cycle change. It also provides independent confirmation of solar
irradiance models which are based on the assumption that irradiance variations
are caused by the evolution of the solar surface magnetic flux. The result
obtained here also supports the spectral irradiance variations computed by
these models.
