  Chromospheric observations of sunspot umbrae offer an exceptional view of
magneto-acoustic shock phenomena and the impact they have on the surrounding
magnetically-dominated plasma. We employ simultaneous slit-based
spectro-polarimetry and spectral imaging observations of the chromospheric He I
10830{\AA} and Ca II 8542{\AA} lines to examine fluctuations in the umbral
magnetic field caused by the steepening of magneto-acoustic waves into umbral
flashes. Following the application of modern inversion routines, we find
evidence to support the scenario that umbral shock events cause expansion of
the embedded magnetic field lines due to the increased adiabatic pressure. The
large number statistics employed allow us to calculate the adiabatic index,
gamma = 1.12 +/- 0.01, for chromospheric umbral locations. Examination of the
vector magnetic field fluctuations perpendicular to the solar normal revealed
changes up to ~200 G at the locations of umbral flashes. Such transversal
magnetic field fluctuations have not been described before. Through comparisons
with non-linear force-free field extrapolations, we find that the perturbations
of the transverse field components are orientated in the same direction as the
quiescent field geometries. This implies that magnetic field enhancements
produced by umbral flashes are directed along the motion path of the developing
shock, hence producing relatively small changes, up to a maximum of ~8 degrees,
in the inclination and/or azimuthal directions of the magnetic field.
Importantly, this work highlights that umbral flashes are able to modify the
full vector magnetic field, with the detection of the weaker transverse
magnetic field components made possible by high-resolution data combined with
modern inversion routines.
