Variation in sunspot properties between 1999 and 2014

Abstract

Aims. We study the variation in the magnetic field strength, area, and continuum intensity of umbrae in solar cycles 23 and 24. Methods. We analyzed a sample of 374 sunspots observed from 1999 until 2014 with the Tenerife Infrared Polarimeter at the German Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter at the Dunn Solar Telescope. The sample of field strength, area, and intensities was used to trace any long-term or cyclic trend of umbral properties in the last 15 years. Results. Sunspots are systematically weaker, that is, have a weaker field strength and stronger continuum intensity, toward the end of cycle 23 than they had at the maximum of cycle 23. The linear trend reverses with the onset of cycle 24. We find that the field strength decreases in the declining phase of cycle 23 by about 112 (±16) G yr −1 , while it increases in the rising phase of cycle 24 by about 138 (±72) G yr −1 . The umbral intensity shows the opposite trend: the intensity increases with a rate of 0.7 (±0.3)% of Ic yr −1 toward the end of cycle 23 and decreases with a rate of 3.8 (±1.5)% of Ic yr −1 toward the maximum of cycle 24. The distribution of the umbral maximum field strength in cycle 24 is similar to that of cycle 23, but is slightly shifted toward lower values by about 80 G, corresponding to a possible long-term gradient in umbral field strength of about 7 ± 4G yr −1 . If instead of the maximum umbral field we consider the average value over the entire umbra, the distribution shifts by about 44 Gauss. Conclusions. The umbral brightness decreases in the rising stage of a solar cycle, but increases from maximum toward the end of the cycle. Our results do not indicate a drastic change of the solar cycle toward a grand minimum in the near future.