  The O9.7 V star HD 54879 is currently the only massive magnetic star whose
magnetic field geometry and rotation period are not constrained. Over the last
three years, we gathered additional observations of this star, obtained using
various instruments at several astronomical facilities with, the aim to
constrain the rotation period and the magnetic field geometry. The new data
include the first full Stokes vector observations with the PEPSI
spectropolarimeter, installed at the Large Binocular Telescope. The acquired
spectropolarimetric observations show a very slow magnetic field variability
related to the extremely slow rotation of HD 54879, which is also indicated in
a dynamical spectrum, displaying variability of the H$\alpha$ line. The most
intriguing result of our study is the discovery of differences in longitudinal
magnetic field strengths measured using different LSD masks containing lines
belonging to different elements. It is the first time that such a differential
analysis of the field strength in dependence of the used lines is carried out
for a magnetic O-type star. Since the LSD Stokes $I$ profiles of the studied O,
Si, and He line masks remain stable over all observing epochs, we conclude that
the detection of different field strengths using lines belonging to these
elements is related to the different formation depths, with the He lines formed
much higher in the stellar atmosphere compared to the silicon and the oxygen
lines, and NLTE effects. Our numerical magnetospherical model suggests the
presence of enhanced gas density that fills the volume inside the field lines
close to the star.
