  Several studies have reported magnetic field fluctuations associated with
umbral shock waves. We aim to study the properties and origin of magnetic field
fluctuations in the umbral chromosphere. Temporal series of spectropolarimetric
observations were acquired with the GREGOR telescope. The chromospheric and
photospheric conditions were derived from simultaneous inversions of the He I
10830 \AA\ triplet and the Si I 10827 \AA\ line using HAZEL2. The oscillations
are interpreted using wavelet analysis and context information from UV
observations acquired with SDO/AIA and IRIS. The chromospheric magnetic field
shows strong fluctuations in the sunspot umbra, with peak field strengths up to
2900 G. Magnetic field and velocity umbral oscillations exhibit a strong
coherence, with the magnetic field lagging the shock fronts detected in the
velocity fluctuations. This points to a common origin of the fluctuations in
both parameters, whereas the analysis of the phase shift between photospheric
and chromospheric velocity is consistent with upwards wave propagation. These
results suggest that the strong inferred magnetic field fluctuations are caused
by changes in the response height of the He I 10830 \AA\ line to the magnetic
field, which is sensitive to high photospheric layers after the shock fronts.
The coronal activity seen in EUV data could possibly have some impact on the
inferred fluctuations, but it is not the main driver of the magnetic field
oscillations since they are found before EUV events take place. Chromospheric
magnetic field fluctuations measured with the He I 10830 \AA\ triplet arise due
to variations in the opacity of the line. After shocks produced by slow
magnetoacoustic waves, the response of the line to the magnetic field can be
shifted down to the upper photosphere. This is seen as remarkably large
fluctuations in the line of sight magnetic field strength.
