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Fig. 11


Two observations of sunspots made with the Very Large Array radio telescope at 15 GHz. In both cases only the image in the sense of circular polarization that minimizes any contribution from gyroresonance emission (the “ordinary” mode) is shown. The upper panels compare a white light image from the MDI instrument on the SOHO satellite (left), showing the umbra and penumbra of a large sunspot observed on 1999 May 13, with a simultaneous 15 GHz image from the VLA in “D” configuration (middle, spatial resolution 5′′). The right panel shows what the VLA would have seen if the only emission were from the sunspot, i.e., the white-light image scaled linearly so that the darkest part of the umbra is 6000 K hotter than the surrounding atmosphere at 15 GHz. The lower panels show another comparison for a sunspot observed on 2001 July 15 when the VLA was in “C” configuration (resolution 2′′). Because the VLA “C” configuration is not sensitive to large structures, the model image in this case does not recover all the enhanced emission from the penumbra, but the umbra is well recovered. The actual and model radio images are displayed with a brightness temperature range of − 5000 to 5000 K relative to the background quiet-Sun level (the VLA is an interferometer and does not measure the absolute level). Contours mark sunspot umbrae and penumbrae.

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