Long-term variations of solar activity
The 11-year solar cycle has been known for centuries. During this time the activity level (measured as sunspot number) changed dramatically from the Maunder minimum (1650-1700) to the Modern maximum in mid 20th century. The extended minimum of the last solar cycle alerted solar physicists about possible long-term variations in the solar magnetic activity. While some argue that the Sun was unusually active in mid 20th century, others find it unusually inactive now. This caused speculations whether the solar activity cycle is overlaid with a long-term decline that may lead to another grand minimum in the near future. Some linear extrapolations predicted that there would be no sunspots in the next cycle. Since solar magnetic activity and space weather have a direct impact on our life, it is important to understand such variations. To this end, we compared the present cycle 24 with the previous one. We measured sunspot intensity, area, and magnetic field strength, seeking for deviations from the previous cycle.
For the present cycle we used the Helioseismic and Magnetic Imager onboard Solar Dynamic Observatory to observe all sunspots between May 2010 and October 2012, once per day. We find nonlinear relations between umbral minimum intensity and size and between maximum magnetic field strength and size. The field strength scales linearly with the intensity and the umbral size scales roughly linearly with the total magnetic flux, while size and field strength level off with larger flux. The slope of the linear fit and the exponent of the power-law fit to the empirical relations (Figure 1) do not differ between cycle 23 and 24.
Although the physical properties of sunspots remained unchanged, the number of sunspots decreased significantly compared to the last cycle. Does this mean that the Sun cannot generate large sunspots and complex active regions? The answer to this question is found in the umbral area distribution function. If there were a lack in the population of large spots, the distribution should deviate from the one in cycle 23. The distribution function of the umbral area in the ascending phase of the current solar cycle is similar to that of the last solar cycle. This means that although the number of sunspots is smaller than the one in cycle 23, the fraction of large umbrae did not change with respect to the previous cycle.
Indeed, the area of active region NOAA 11967 (Feb 03-04, 2014, Figure 2) was about 2172 millionths of the hemisphere (MHS*), the largest one since many years, slightly larger than the famous big sunspot group NOAA 9169 (Sep 22, 2000) with an area of 2140 MHS.
From our comparison of umbral area, magnetic field, and umbral intensity of the sunspots of the rising phase of cycle 24 with the previous cycle, we do not find a significant indication of cycle-to-cycle changes in either sunspot physical properties or the distribution of sunspot umbral area. This is in agreement with our earlier findings (R. Rezaei, C. Beck, & W. Schmidt 2012, A&A, 541, A60).
(*) The size of sunspots is often given in “millionth hemispheres of the Sun” (MHS). An area of 100 MHS corresponds to 300 Million square kilometers, or a round spot with a diameter of 20.000 km (27 arcsec).