Abstract
The minimum intensities of sunspot umbrae were measured from 1992 through 2003 using the National Solar Observatory Kitt Peak Vacuum Telescope spectromagnetograph data. The resulting 3931 umbral measurements reveal a solar cycle variation in the umbral intensity, with umbrae appearing brighter on average at cycle minimum and darker at cycle maximum. Recent studies of umbral intensity have been in direct conflict with each other, showing decreasing, increasing, or no umbral intensity trends. By providing a longer temporal baseline, our new measurements uncover a solar cycle oscillation in umbral intensity. From this new perspective we show how previous studies agree with our work and with each other when the same phase of the solar cycle is examined. Also in agreement with other investigators, the solar cycle variation in the daily average umbral area, the lognormal distribution of umbral area, and the near constancy of the mean umbral radius during the solar cycle are seen in this data.
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