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Fig. 1
a) Centre-of-gravity magnetogram Fe 617.3 nm (absolute value). b) Amplitude of 3 mHz oscillations (Fe 617.3 nm). c) Amplitude of 5 mHz oscillations (Fe 617.3 nm). d) Position in frequency of the power spectrum peak (Fe 617.3 nm). The contour indicates the position of the umbra as seen in intensity images.
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