Long-Term Variations of the Magnetic Field of the Solar Photosphere

Variations of weak magnetic fields of the photosphere with periods on the order of the solar magnetic cycle have been studied. Synoptic maps of the photospheric magnetic field obtained by NSO Kitt Peak for the period from 1978 to 2016 were used as initial data. In order to study weak magnetic fields, the saturation threshold for synoptic maps was set equal to 5 G. The converted synoptic maps were used to construct the time–latitude diagram; 18 temporal profiles of the magnetic field for latitudes uniformly distributed along the sine from the north to the south pole are highlighted and analyzed in the diagram. The time dependences were averaged by sliding smoothing over 21 Carrington rotations. Approximation of the time dependences by a sinusoidal function made it possible to distinguish a cyclic component with a period of about 22 years (the period of the Hale magnetic cycle) in weak magnetic fields. The dependence of the 22-year variation on latitude is considered. In addition to the well-known 22-year change in the near-polar field, similar variations were found for fields at all latitudes. The variation amplitude decreased from the poles to the equator, while the variation period remained almost constant (T = 22.3 years). The exceptions were latitudes 26° and 33° in the northern and 26° in the southern hemisphere. These intervals of middle latitudes were distinguished by the predominance of short-period variations.