  Quasi-periodic changes of the paleointensity and geomagnetic polarity in the
intervals of 170 Ma to the present time and of 550 Ma to the present time were
studied, respectively. It is revealed that the spectrum of the basic variations
in the paleointensity and of the duration of the polar intervals is discrete
and includes quasi-periodic oscillations with characteristic times of 15 Ma, 8
Ma, 5 Ma, and 3 Ma. The characteristic time of these quasi-periodic changes of
the geomagnetic field at the beginning and at the end of the Phanerozoic
differed by no more than 10%. The spectral density of quasi-periodic variations
of the geomagnetic field changed cyclically over geological time. The relation
between the behaviors of the amplitude of paleointensity variations, the
duration of the polar intervals, and their spectral density was shown.
Quasi-periodic variations of the paleointensity (geomagnetic activity) had a
relatively high spectral density in the interval of (150 - 40) Ma (in the
Cretaceous - Early Paleogene). In this interval, both the amplitude of
paleointensity variations and the duration of polar intervals increased. In the
intervals of (170 - 150) Ma and of 30 Ma to the present, a quasi-periodic
variation in the paleointensity practically did not detect against the
background of its noise variations. At the same time, the amplitude of the
paleointensity variations and duration of polar intervals decreased. An
alternation of time intervals in which the paleointensity variations acquired
either a quasi-periodic or noise character took place during the geomagnetic
history.

Geophysics