The Centennial Gleissberg Cycle and its association with extended minima

Abstract The recent extended minimum of solar and geomagnetic variability (XSM) mirrors the XSMs in the nineteenth and twentieth centuries: 1810–1830 and 1900–1910. Such extended minima also were evident in aurorae reported from 450 A.D. to 1450 A.D. This paper argues that these minima are consistent with minima of the Centennial Gleissberg Cycles (CGCs), a 90–100 year variation observed on the Sun, in the solar wind, at the Earth, and throughout the heliosphere. The occurrence of the recent XSM is consistent with the existence of the CGC as a quasiperiodic variation of the solar dynamo. Evidence of CGCs is provided by the multicentury sunspot record, by the almost 150 year record of indexes of geomagnetic activity (1868 to present), by 1000 years of observations of aurorae (from 450 to 1450 A.D.) and millennial records of radionuclides in ice cores. The aa index of geomagnetic activity carries information about the two components of the solar magnetic field (toroidal and poloidal), one driven by flares and coronal mass ejections (related to the toroidal field) and the other driven by corotating interaction regions in the solar wind (related to the poloidal field). These two components systematically vary in their intensity and relative phase giving us information about centennial changes of the sources of solar dynamo during the recent CGC over the last century. The dipole and quadrupole modes of the solar magnetic field changed in relative amplitude and phase; the quadrupole mode became more important as the XSM was approached. Some implications for the solar dynamo theory are discussed.