Cyclic and Long-Term Variation of Sunspot Magnetic Fields

Abstract

Measurements from the Mount Wilson Observatory (MWO) are used to study the long-term variations of sunspot field strengths from 1920 to 1958. Following a modified approach similar to that in Pevtsov et al. (2011), for each observing week we select a single sunspot with the strongest field strength measured that week and then compute monthly averages of these weekly maximum field strengths. The data show the solar cycle variation of the peak field strengths with an amplitude of about 500-700 gauss (G), but no statistically significant long-term trends. Next, we use the sunspot observations from the Royal Greenwich Observatory (RGO) to establish a relationship between the sunspot areas and the sunspot field strengths for Cycles 15-19. This relationship is then used to create a proxy of peak magnetic field strength based on sunspot areas from the RGO and the USAF/NOAA network for the period from 1874 to early 2012. Over this interval, the magnetic field proxy shows a clear solar cycle variation with an amplitude of 500-700 G and a weaker long-term trend. From 1874 to around 1920, the mean value of magnetic field proxy increases by about 300-350 G, and, following a broad maximum in 1920-1960, it decreases by about 300 G. Using the proxy for the magnetic field strength as the reference, we scale the MWO field measurements to the measurements of the magnetic fields in Pevtsov et al. (2011) to construct a combined data set of maximum sunspot field strengths extending from 1920 to early 2012. This combined data set shows strong solar cycle variations and no significant long-term trend (linear fit to the data yields a slope of −0.2±0.8 G year −1 ). On the other hand, the peak sunspot field strengths observed at the minimum of the solar cycle show a gradual decline over the last three minima (corresponding to cycles 21-23) with a mean downward trend of ≈ 15 G year −1 .

An Observed Decline in the Amplitude of Recent Solar-Cycle Peaks

“…A set of unbiased measurements of umbral contrast indicates that umbral contrast has not been decreasing with time, but has remained constant (Leonard and Choudhary, 2008;de Toma et al, 2013). This is consistent with the results of Pevtsov et al (2014), who found no significant long-term trend in the maximum sunspot magnetic-field strength. Another complication is suggested by the finding that the relation between spot number and F 10.7 flux has been changing (Livingston, Penn, and Svalgaard, 2012) while the relation between F 10.7 and sunspot area (de Toma et al, 2013) and F 10.7 magnetic indices (Henney et al, 2012) has remained the same.…”

“…The average cycle maximum in sunspot number for Cycles 22, 23, and 24 is about 150, 110, and 60, respectively (www.sidc.oma.be). On the other hand, Pevtsov et al (2014) concluded from a detailed study of sunspot magnetic fields that there has been no secular decline in the maximum magnetic field strength in sunspots, except perhaps during the last few minima. The decline in peak activity could be part of a longer-term cycle such as the Gleissberg cycle (Gleissberg, 1967;Usoskin and Mursula, 2003;Mouradian, 2013;McCracken et al, 2013).…”

The Discovery of Solar-Like Activity Cycles Beyond the End of the Main Sequence?

“…If a global dipolar field exists and reverses direction, this may be detectable by temporal changes in the cutoff frequency of the radio emission over month-to-year long timescales. For example, the annual averages of sunspot magnetic field strengths vary in accordance with the solar cycle, with stronger fields at solar maximum that then decline until solar minimum, when magnetic reversal occurs (Pevtsov et al 2014). A study of the monthly mean total stellar radio flux offers another means of assessing the presence of activity cycles in brown dwarfs, as this quantity varies in a prominent way during the solar activity cycle for 1-9.6 GHz frequencies (Shibasaki 2011).…”

Variation in sunspot properties between 1999 and 2014

“…Livingston et al (2006) and Penn & Livingston (2011) found that the field strength of sunspots was stronger at the beginning of cycle 22 than at its end. Rezaei et al (2012), Pevtsov et al (2014), and Schad (2014) reported a cyclic variation of the field strength, but found no significant trace of a longterm variation in their data. Livingston et al (2012) predicted a monotonic decrease of sunspot field strength that should lead to the disappearance of sunspots in cycle 25.…”