Abstract

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. We examine a number of other solar activity indicators 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 that vary in association with the sunspots. We examine the characteristics of individual solar cycles including their maxima and minima, cycle periods and amplitudes, cycle shape, and the nature of active latitudes, hemispheres, and longitudes. We examine long-term variability including the Maunder Minimum, the Gleissberg Cycle, and the Gnevyshev-Ohl 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.

Highlights

  • Solar activity rises and falls with an 11-year cycle that affects us in many ways

  • The Boulder Sunspot Number is derived from the daily Solar Region Summary produced by the US Air Force and National Oceanic and Atmospheric Administration (USAF/NOAA) from sunspot drawings obtained from the Solar Optical Observing Network (SOON) sites since 1977

  • This indicates that north–south asymmetry can persist for years

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Summary

Introduction

Solar activity rises and falls with an 11-year cycle that affects us in many ways. Increased solar activity includes increases in extreme ultraviolet and x-ray emissions from the Sun which produce dramatic effects in the Earth’s upper atmosphere. Increases in the number of solar flares and coronal mass ejections (CMEs) raise the likelihood that sensitive instruments in space will be damaged by energetic particles accelerated in these events. These solar energetic particles (SEPs) can threaten the health of both astronauts in space and airline travelers in high altitude, polar routes. The existence of spots on the Sun came as a surprise to westerners when telescopes were first used to observe the Sun in the early 17th century. One can only hope that, with frequent observations of periodic motion of space objects, that time will show how to examine in which way astronomical bodies that are driven and lit up by the Sun are influenced by sunspots.” (Wolf, 1877, translation by Elke Willenberg)

Schwabe’s discovery
Wolf ’s relative sunspot number
Wolf ’s reconstruction of earlier data
Sunspot numbers
Sunspot areas
Total irradiance
Magnetic field
Flares and Coronal Mass Ejections
Geomagnetic activity
3-9 X Flares
Cosmic rays
Radioisotopes in tree rings and ice cores
Minima and maxima
Smoothing
Cycle periods
Cycle amplitudes
Cycle shape
Active latitudes
Active hemispheres
4.10 Active longitudes
The Maunder Minimum
The secular trend
The Gleissberg Cycle
Long-term variations from radioisotope studies
The Suess cycle
Quasi-biennial variations and double peaked maxima
Predicting an ongoing cycle
Predicting future cycle amplitudes based on cycle statistics
Predicting future cycle amplitudes based on geomagnetic precursors
Predicting future cycle amplitudes based on dynamo theory
Findings
Conclusions

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