Long-period magnetic fluctuations and mantle electrical conductivity estimates

Abstract

Most attempts to use the worldwide magnetic fluctuations to study the earth's conductivity structure have been concentrated on the 24-hour variations and the magnetic storm transients. Some of these results have been inconsistent, possibly because of the ocean effect and the complex mode structure of the variations. Longer-period fluctuations, considered to be due to modulations of the storms, are less subject to these difficulties. Spectral analyses of magnetic station data have been made in an attempt to find such variations. The 11-year variations produce too few vertical variations to be distinguished from the internal variations and the general noise level, but strong signals were obtained at 13.5-day periods and some data from periods as long as 6 months. Mantle conductivity profiles are tested in relation to these data, and the K. McDonald model appears quite adequate to the depth sampled by these frequencies.