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. These data show agreement with three other umbral intensity studies, and the similarity with infrared spectroscopic measurements suggests that the mean umbral maximum magnetic field also varies through the solar cycle by 600 Gauss. In agreement with other investigators, the solar cycle variation in the daily average umbral area, the log-normal distribution of umbral area, and the near constancy of the mean umbral radius during the solar cycle are seen in this data.
Authors: MJ Penn, RKD MacDonald
Publication Status: ApJ submitted
Last Modified: 2007-02-14 10:33