Variations in the Auroral Activity and Main Magnetic Field of the Earth over 300 years (1600–1909)

Abstract

We estimate the contribution of the systematic component of the Earth’s main magnetic field (MMF), i.e., the main dipole, to the secular variation in the midlatitude polar auroras based on the archived data. It is found that the main features of the variations in the magnetic moment (MM) of the main dipole in 1600–1909 are reflected in variations in the annual number of polar auroras N with the opposite sign (correlation coefficient of 0.6–0.8). A model of the dependence of the number of polar auroras N on the solar activity expressed in the Wolf number W and on the MM value is proposed. Taking into account the effect of the MM noticeably improves the properties of the N versus W dependence. It is found that the disagreement between the 11-year cycles of N and W observed in 1700–1775, as well as the minimum N in ~1760–1767 (Silverman minimum), are conditioned by MM variations in the corresponding periods of time. A rapid increase in the MM near 1800 significantly contributes to the steep decline in N during the Dalton minimum. During those historical periods in which the MM value was 1.5–2 times greater than in the 17th–19th centuries (according to the archeomagnetic data), the number of polar auroras could be conditioned not so much by the solar activity as by the screening effect of the MMF.