Magnetic field variations associated with umbra! flashes and penumbral waves

Context. Umbral flashes (UF) and running penumbral waves (RPWs) in sunspot chromospheres leave a dramatic imprint in the intensity profile of the Can 8542 angstrom line. Recent studies have focussed on also explaining the observed polarization profiles, which 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. These changes could be explained by opacity effects or by intrinsic changes in the magnetic field strength. Aims. 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 Can 8542 angstrom 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 the magnetic field, B. Methods. We have performed non local thermodynamical equilibrium (non-LTE) data inversions with the NICOLE code of a timeseries of very high spatio-temporal-resolution observations in the Can 8542 angstrom, Fei 6301.5, and Fei 6302.5 angstrom lines. We analyze in detail the variations of the different physical parameters of the model as a function of time. Results. Our results indicate that the Can 8542 angstrom line in sunspots is greatly sensitive to magnetic fields at log tau(500 )= -5 (hereafter log tau = -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 (up to similar to 270 G) and in RPWs (similar to 100 G). Conclusions. 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.