New features observed in the 11-year solar cycle

Abstract

The number of sunspots is characterized by a long-term temporal variation, reaching its maximum or its minimum approximately every 11 years (the solar cycle). This variation, in turn, has an effect in terms of variation in the global climate. Since 1979, the use of satellite-borne radiometers has enabled accurate measurements of total solar irradiance (TSI). For instance, the sunspot numbers that are scaled to correspond to Nimbus-7 TSI observations for 1979–1993 show little long-term trend. However, while the observations of different extremes of the solar cycle, which are available from 1749, seem irregular in time and magnitude and difficult to quantify, they appear to have a strong correlation between them when they are sorted pairwise according to their size rather than sequentially in time. A similar relation holds among the solar cycle lengths (periods) and the solar cycle extremes, which, in parallel, obey a linear relation that is reminiscent of the Gutenberg–Richter seismic law. This can be used for a probabilistic approach to forecast solar parameters that are connected to global climate.

Based on the reasonable assumption that the basic parameters like extremes and length of the 11-year solar cycle are associated with the energy oscillating between the dipolar and quadrupolar phases of the cycle, it is concluded that these parameters obey a power-law distribution similar to that of the Gutenberg–Richter seismic law.

The question of whether solar activity is deterministically chaotic is also investigated by exploring the behaviour of the main characteristics of the 11-year solar cycle. This is done by constructing return maps of solar cycle strength and duration, which seem to take the familiar up–down U shape, implying both non-linearity and re-injection. The results suggest that there might exist a coupling between two or three different non-linear deterministic dynamical systems on the Sun, depending on which variable of the solar cycle is being considered.

Acknowledgments

The authors wish to thank Mr Tzanis Christos and Mrs Efstathiou for their help in the calculations.