The Contribution of Large Recurrent Sunspot Groups to Solar Activity: Empirical Evidence

We identify large sunspot nestlets (SN) mostly containing recurrent sunspot groups and investigate the indices of solar activity defined as the 11- or 22-year moving average of the daily areas of the SN. These nestlets, 667 in total, are constructed from the daily 1874–2020 RGO/SOON catalogue, which contains 41,394 groups according to their IDs, with a machine-learning technique. Within solar cycles 15–19, the index contributed disproportionately strongly to the overall solar activity: the index is normalized to a quasi-constant shape by a power function of the activity, where the exponent is approximately 1.35. Large SN contribute to solar activity even more in cycle 22, underlying the second largest peak of solar activity within the last Gleissberg cycle in ∼1985. Introducing another composite, moderate SN normalized by the overall activity, we observe its quasi-constant shape in cycles 15–19 and a general anti-correlation with the first normalized composite. The constructed sunspot nestlets constitute a modified catalogue of solar activity. We define the average lifetime per day in 22-year windows for the modified catalogue, in line with Henwood et al. (SoPhys 262, 299, 2010), and reproduce the dynamics of this quantity they revealed for 1900–1965. The average lifetime derived from the moderate SN is found to form a wave with minima at the beginning of the 20th and 21st centuries, resembling the Gleissberg cycle with long minima. The average lifetime characterizing large SN exhibited a deeper minimum at the beginning of the 20th century than 100 years later.

The Solar Cycle

The solar cycle is reviewed. The 11-year cycle of solar activity is characterized by the rise and fall in the numbers and surface area of sunspots. A number of other solar activity indicators also vary in association with the sunspots including; the 10.7 cm radio flux, the total solar irradiance, the magnetic field, flares and coronal mass ejections, geomagnetic activity, galactic cosmic ray fluxes, and radioisotopes in tree rings and ice cores. Individual solar cycles are characterized by their maxima and minima, cycle periods and amplitudes, cycle shape, the equatorward drift of the active latitudes, hemispheric asymmetries, and active longitudes. Cycle-to-cycle variability includes the Maunder Minimum, the Gleissberg Cycle, and the Gnevyshev-Ohl (even-odd) Rule. Short-term variability includes the 154-day periodicity, quasi-biennial variations, and double-peaked maxima. We conclude with an examination of prediction techniques for the solar cycle and a closer look at cycles 23 and 24.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material is available for this article at 10.1007/lrsp-2015-4.

Using the dipolar and quadrupolar moments to improve solar-cycle predictions based on the polar magnetic fields

The solar cycle and its associated magnetic activity are the main drivers behind changes in the interplanetary environment and Earth's upper atmosphere (commonly referred to as space weather and climate). In recent years there has been an effort to develop accurate solar cycle predictions, leading to nearly a hundred widely spread predictions for the amplitude of solar cycle 24. Here we show that cycle predictions can be made more accurate if performed separately for each hemisphere, taking advantage of information about both the dipolar and quadrupolar moments of the solar magnetic field during minimum.