  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 10.7cm 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.
