  Variations of the weak magnetic fields of the photosphere with periods of the
order of the solar magnetic cycle were investigated. Synoptic maps of the
photospheric magnetic field produced 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 at 5 G. On the base of
transformed synoptic maps the time-latitude chart was built. 18 profiles of the
magnetic field evenly distributed along the sine of latitude from the north to
the south pole were selected in the diagram for the further analysis. Time
dependencies were averaged by sliding smoothing over 21 Carrington rotations.
The approximation of averaged time dependencies by the sinusoidal function made
it possible to distinguish in weak magnetic fields a cyclic component with a
period of about 22 years (the period of the Hale magnetic cycle). The
dependence of 22-year variation on latitude was studied. In addition to the
well-known 22-year change in the near-polar field, similar variations were
found for the fields at all latitudes. The exception was latitudes $26^\circ$
and $33^\circ$ in the northern and $26^\circ$ in the southern hemisphere. These
mid-latitude intervals were characterized by a predominance of short-period
variations. The amplitude of the long-term variation decreased from the poles
to the equator, with the period of variation remaining almost constant (T =
22.3 years).
