Context. Umbral chromospheric observations show the presence of magnetoacoustic shocks. Several recent studies have reported magnetic field fluctuations associated with these shock waves. The mechanism behind these periodic magnetic field changes is still an unresolved question.
Aims: We aim to study the properties and origin of magnetic field fluctuations in the umbral chromosphere.
Methods: Temporal series of spectropolarimetric observations were acquired with the GREGOR telescope on 2017 June 18. The chromospheric and photospheric conditions, including the temporal evolution of the magnetic field, were derived from simultaneous inversions of the He I 10 830 Å triplet and the Si I 10 827 Å line using the HAZEL2 code. The oscillations were interpreted using wavelet analysis and context information from ultraviolet observations acquired with the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO/AIA) and the Interface Region Imaging Spectrograph (IRIS).
Results: The chromospheric magnetic field shows strong fluctuations in the sunspot umbra, with peak field strengths of up to 2900 G. This inferred field strength is comparable to the magnetic field strength in the upper photosphere. 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, and the analysis of the phase shift between photospheric and chromospheric velocity is consistent with upward wave propagation. These results suggest that the strong inferred magnetic field fluctuations are caused by changes in the response height of the He I 10 830 Å line to the magnetic field, which is sensitive to the high photosphere during and right after the shock. The analysis of extreme ultraviolet data shows a weak brightening in a coronal loop rooted in the umbra around the time of the measured magnetic field fluctuations. This coronal activity 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 the extreme ultraviolet event takes place.
Conclusions: Chromospheric magnetic field fluctuations measured with the He I 10 830 Å triplet arise due to variations in the opacity of the line. After strong shocks are produced by the propagation of 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.