Vector of the geomagnetic field F is usually presented by 3 components: horizontal (H), vertical (Z) and declination (D) components. Declination is the clockwise angle between the horizontal component of the geomagnetic field and the geographic north line.

Secular variation of D shows that the magnetic meridian in Irkutsk has shifted from +2º 13’ to –2º 22’ relative to the geographic meridian over the last 116 years. The magnetic compass needle pointed the geographic North Pole only in 1932. Secular variation of D has not yet revealed any periodic process. Abrupt changes in the secular variation (1921 – 1931) may be caused by the measurement errors resulting from the fact that many specialists were laid off for political reasons.

The diagram presents both wave-like increase in full vector F by 1700 nT and periodicity of the F secular variation with the period of 50 years. This conclusion is borne out by measurements of vertical component Z which is a part of vector F (see diagram 3). Short-term changes in the secular variation before 1947 were mainly due to instrumental errors in dip-needles and induction inclinometers.

Secular variation of the horizontal component shows gradual wave-like decrease in Н by 1250 nT; preliminary estimate of the period of the Н secular variation is 80 years. If we go on observations of the geomagnetic field, this assumption will be either substantiated or disproved in the next century. Changes in the Н secular variation depend mainly on the number of magnetic storms within a year (i.e., they are related to solar activity).