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We investigate the effect of the Joule heating (JH) mechanism on the brightness of umbral dots (UDs), proposed by Hamedivafa (2003, A&A, 407, 761). Time series of high-resolution images of UDs in a developed sunspot and in a large pore are used for this purpose. The effect of the JH mechanism is characterized by a specific shape of the temporal variations of UD brightness and area and it is observed better in those UDs where the magnetic field strength is very weak compared to their surroundings. In our observations, about 12% of UDs in the sunspot and about 14% of UDs in the pore show indications of the effective presence of the JH power.

Observational evidence of Joule heating in some umbral dots