Christoph Kiess, Reza Rezaei,and Wolfgang Schmidt
Kiepenheuer-Institut für Sonnenphysik (KIS), Schöneckstr. 6, 79104, Freiburg, Germany

The 11-year solar magnetic activity cycle has been observed by telescope for the last four centuries. The activity level changed dramatically from the Maunder minimum (1650-1700) to the Modern maximum in mid 20th century. The extended minimum that occurred after cycle 23 alerted solar physicists to the possible onset of a long-term decline in magnetic activity. While some argue that the Sun was unusually active in mid 20th century [1], others find it unusually inactive now [2]. 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 predicted that there will be no sunspots produced in the next cycle. Since solar magnetic activity and space weather can directly impact our lives through a dependency upon technology such as telecommunications and electricity networks, it is important to understand such sunspot cycle extremes. To this end, we present measurements of sunspot intensity, area, and magnetic field strength from the present cycle 24 to compare with the previous cycle 23 [3] and determine if physical properties of sunspots deviate from the previous cycle.

We used HMI / SDO to observe all sunspots between May 2010 – Oct 2012, once per day. We created two subsets of this data with a manual tracking algorithm. The first sample contains 205 fully evolved sunspots, primarily leading polarity spots in active regions shortly after the spot reached its maximum size. Spots in this sample have a simple morphology and are close to disk-center. Using an inversion of the full Stokes observations from HMI, we retrieved the magnetic field strength of these spots. 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. The umbral size scales roughly linearly with the total magnetic flux, while the size and field strength level off with larger flux. The slope of the linear fit and exponent of the power-law fit to the empirical relations (Fig 1) agree with similar results in cycle 23 [4]. Therefore, properties of individual sunspots are comparable between cycle 24 and 23.

Figure 1 | Left: Variation of umbral minimum intensity vs. radius. Middle: Variation of umbral maximum field strength as a function of radius. The yellow line shows a linear fit to measurements of six sunspots in cycle 22 by Kopp & Rabin (1992), while the violet line marks observations of Schad & Penn in the rising phase of cycle 23 (2010). Right: Maximum field strength of umbra vs. minimum intensity. In all panels, red and blue lines mark a linear and power-law fit, respectively. The dashed lines mark the one sigma confidence level for each fit.

It is known that the two hemispheres are temporally lagged in their activity by 6 months or so. When we separate the 205 sunspots by hemisphere and averaged in time, the southern hemisphere shows a temporal increase in umbral size while there is no significant trend in the northern hemisphere. Also there is no temporal variation in the umbral mean intensity detectable in either hemisphere. Perhaps the duration of our study should be extended to facilitate the detection of a weak secular trend, namely variation of umbral properties as a function of the solar cycle phase.

Even if the physical properties of sunspots remained intact, the number of sunspots decreased significantly compared to last cycle. Does this mean that the Sun cannot generate large sunspots and complex active regions? To answer this question, we plotted the distribution function of umbral size. If there was a lack in population of small or large spots, the distribution should deviate from the one in cycle 23. To this end, we created a second sample which has 910 umbrae out of 6892 individual umbra in our data (we counted each sunspot once). 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 number of sunspots are 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, see video) was about 2172 millionths of the hemisphere, which is slightly larger than the famous sunspot group NOAA 9169 from Sep 22, 2000 (http://sohowww.nascom.nasa.gov/hotshots/2000_09_22/) with an area of 2140 millionths of the visible solar surface.

A movie of the very large active region, NOAA 11967, is shown as it crosses the disk from Jan 29 – Feb 7 2014. The movie was generated using Jhelioviewer.

We analyze umbral area, magnetic field, and umbral intensity of the sunspots during the rising phase of cycle 24. We do not find a significant variation in either sunspot physical properties or distribution of sunspot umbral area. This agrees with an earlier finding that even if a long-term trend exists, its amplitude is smaller than what can be detected in the cyclic variation of umbral properties [5].

References

[1] Usoskin, I.G., Solanki, S.K., Schüssler, M., et al. 2003, Phys Rev Let, 91, 211101
[2] Livingston, W., Penn, M. J., & Svalgaard, L. 2012, ApJ, 757, L8
[3] Kiess, C., Rezaei, R., & Schmidt, W. 2014, A&A, accepted http://arxiv.org/abs/1402.2881
[4] Mathew, S.K., Lagg, A., Solanki, S.K., et al. 2003, A&A, 410, 695
[5] Rezaei, R., Beck, C., & Schmidt, W. 2012, A&A, 541, A60