Reflection of the solar activity Hale cycles on the geomagnetic field secular variations

Abstract

The effect of dramatic decaying of solar activity in the twentieth cycle on secular variations of the Earth’s geomagnetic field has been investigated. Solar activity decay leads to reduction of the number of magnetic storms on the Earth. During the years of large-scale solar activity negative variations from external sources of the field — circular magnetospheric current, increase, as well as amplitudes of changes of auroral and equatorial ionospheric electrojets. Circular current reduces the horizontal component of the Earth’s magnetic field in low and medium latitudes and increases the vertical component in the high latitudes. In auroral latitudes the intensity of auroral electrojets increases during a magnetic storm. Therefore in more active years average annual absolute values of horizontal component of magnetic field will be smaller at the observatories situated in low and medium latitudes and average annual absolute values of the vertical component of magnetic field will be higher in the high latitudes of the Earth. As far as secular variations are determined as the difference between average annual values of the field, the effect of external sources of the field increases with the growth of solar activity.At the observatories of the northern hemisphere of the Earth (West Europe and North America) decaying of solar activity leads to the increase of average annual values of the field, i.e., to positive secular variations. The effect is maximal in the region of the magnetic pole. The amplitude of the effect decreases with the decrease of the latitude of the observatory. The sign of the effect changes (i.e. negative values of secular variations for the northern hemisphere) at the longitudes 60°Е («Kazan») and 150—160°E («cape Uellen», «Barrov»). Amplitudes of negative secular variations are maximal at the magnetic observatory «Irkutsk». The effect is not observed at the insular observatories as well as the observatories in South America. It is evidently a result of superposition of variations with different signs from the external and internal sources such as variations from the currents in the oceanic flows and the equality by the value of amplitude of secular variations from external and internal sources.