Secular trend near the focus of maximum horizontal intensity

Abstract

Secular variations inH at four Indian observatories near the regional focus of maximum horizontal intensity show that a reversal of the increasing trend occurred from about, 1965. Broad agreement in the secular trends is seen with some differences in detail. The changes observed at the observatories do not appear to be due to positional shift of the focus. The differences in the trends are attributable to secondary induced current effects in the earth's upper mantle or crust. An apparent long-period variation of 110 years is shown to be part of a much longer period trend.