The geomagnetic spectrum-40 days to 5.5 years

Abstract

Mean daily values of magnetic data for 27 observatories have been digitized and power spectrums computed; periods greater than 40 days are discussed. The mean amplitudes of the semiannual line are 5.29 γ for H and 2.41 γ for Z, while the respective values for the annual line are 4.80 γ and 4.75 γ. Amplitude, coherence, and phase observations indicate the lines have fundamentally different generating mechanisms. A ring current origin, probably situated near 3 RE is proposed for the semiannual line. Recent observations support an early theory of Vestine that meridional ionospheric winds blowing from the summer toward the winter hemisphere are the origin of the annual line. A previously reported quasi-biennial line has not been found, but evidence suggests the statistical reality of the solar cycle subharmonic. Geomagnetic north (X′) and east (Y′) spectrums have resolved the semiannual and annual Y′ line. The existence of a semiannual Y′ line supports satellite evidence for meridional currents in the magnetosphere. The coherence and phase for the continuum indicate that the entire continuum spectrum from 40 days to 3.7 years is primarily due to storm modulations.