Abstract:
Spherical-harmonic-Fourier (SHF) analysis of sunspot activity during 1902-54 show that the sunspot occurrence probability p(theta, phi, t) is given by superposition of, mainly, axisymmetric even degree SHF modes of l not greater than 22, all having periods about 11 yr. The amplitudes and the phases of these modes are found to remain approximately constant over the five cycles, but the small variations in the phases seem to be systematic and coherent. The combination of the amplitudes and the phases determined is unique in reproducing the main qualities of the butterfly diagrams. A 'nominal toroidal field', defined in terms of p(theta, phi, t), yields axisymmetric modes of odd degrees, l not greater than 13, and periods of about 22 yr. The amplitude spectrum of these modes is similar to that of the 'magnetic' modes obtained recently by Stenflo (1988) from the 'radial' fields observed during 1960-86. Reasons are given for believing that these 'magnetic' modes represent global slow MHD oscillations of the sun.
