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We investigate the signal which is expected to be produced by magnetic field oscillations in sunspots umbrae due to the combination of the oscillation model, radiative transfer and observing procedure. For this purpose we investigate the signal expected to be produced by theoretical models of sunspot oscillations and compare them with the signal seen in observed power spectra of sunspot magnetograms. We show that the amplitudes of the observed oscillations are compatible with the predictions of the theoretical model of magnetoacoustic oscillations for the 5-min as well as for the 3-min band. For the particular sunspot umbral oscillation models used, our analysis suggests that most of the expected observed power in the magnetogram signal oscillations is actually due to cross-talk from the temperature and density oscillations associated with the magnetoacoustic wave. A detailed modelling of the observing procedure turns out to be of central importance for the assignment of the observed oscillations to a specific wave type.

A comparison between model calculations and observations of sunspot oscillations