  Umbral flashes (UF) and running penumbral waves (RPWs) in sunspot
chromospheres leave a dramatic imprint in the intensity profile of the Ca II
854.2 nm line. Recent studies have focussed on also explaining the observed
polarization profiles, that show even more dramatic variations during the
passage of these shock fronts. While most of these variations can be explained
with an almost constant magnetic field as a function of time, several studies
have reported changes in the inferred magnetic field strength during UF phases.
In this study we investigate the origin of these periodic variations of the
magnetic field strength by analyzing a time-series of high temporal cadence
observations acquired in the Ca II line with the CRISP instrument at the
Swedish 1-m Solar Telescope. In particular, we analyze how the inferred
geometrical height scale changes between quiescent and UF phases, and whether
those changes are enough to explain the observed changes in $B$. We have
performed non-LTE data inversions with the NICOLE code of a time-series of very
high spatio-temporal resolution observations in the Ca II and Fe I
630.15\630.25 nm lines. Our results indicate that the Ca II line in sunspots is
greatly sensitive to magnetic fields at $\log\tau_{500}=-5$ during UFs and
quiescence. However, this optical depth value does not correspond to the same
geometrical height during the two phases. Our results indicate that during UFs
and RPWs the $\log\tau=-5$ is located at a higher geometrical height than
during quiescence. Additionally, the inferred magnetic field values are higher
in UFs (~270 G) and in RPWs (~100 G). Our results suggest that opacity changes
caused by UFs and RPWs cannot explain the observed temporal variations in the
magnetic field, as the line seems to form at higher geometrical heights where
the field is expected to be lower.
