Terrestrial‐magnetic activity and its relations to solar phenomena

Abstract

A homogeneous series of monthly means of terrestrial‐magnetic activity for the years 1872 to 1930 is derived and extended backward, in annual means, to 1835. The annual variation of magnetic activity and of the relative sunspot‐numbers is discussed by means of new tests for periods. Only the semi‐annual wave in magnetic activity is recognized as physically significant. Its maxima prefer the times when the Sun is in the celestial equator, and not, as has been suggested, the times when the Sun's axis is most inclined towards the ecliptic. This view is supported by tests using the harmonic dial and the probable‐error circle, and several independent considerations.The close relations between sunspot‐numbers and terrestrial‐magnetic activity in the annual and monthly means are discussed. Some general statistical aspects are given for the treatment of the correlation between such series with after‐effects, for which both solar activity and terrestrial‐magnetic activity are typical. The homogeneity of the whole available series for relative sunspot‐numbers and for areas of sunspots and faculae is tested; some inhomogeneities are found, apart from a general lag of terrestrial‐magnetic activity that has occurred in some sunspot‐cycles. A break in the homogeneity of the international magnetic character‐figures in recent years is discovered.The individual 27‐day recurrences in terrestrial‐magnetic activity during 1906–31, and their relations to solar activity are discussed with the help of a graphical day‐by‐day record. They indicate the existence of persistent active areas on the Sun's surface, called M‐regions, which, in many cases, cannot be coordinated to such solar phenomena as are observable by direct astrophysical methods. This holds in particular for the new solar indices which are available for the years 1928–30, and which are found so closely correlated to sunspot‐numbers, that they fail to improve the correlation between solar activity and terrestrial‐magnetic activity. Observations of terrestrial‐magnetic activity yield therefore not only information about geophysical influences of such solar phenomena that may be traced in astrophysical observations, but supplement these direct observations themselves.