Some preliminary results of diurnal variation of magnetic inclination at field stations in South America

Abstract

The results of some of the field determinations of diurnal variation of inclination by means of observations with the earth inductor are reported; these show interesting characteristics at some of the stations along the Pacific coast of Peru and Chile. On January 27, 1917, which is one of the ten least disturbed days for the month, a total diurnal range of inclination of nearly 20′ was observed at Arequipa, Peru; observations in 1924 to 1926 show similar though somewhat smaller ranges at the same station and at other stations in the Andean region. An attempt to relate this unusual daily range to the altitude of the stations or to the nearness of high mountains has yielded only negative results. A method adopted for analyzing these partial daily curves by modification of Fourier series is described and illustrated; the resulting coefficients, though depending on Certain arbitrary assumptions, are useful in comparing curves from different stations and for transferring results to other stations where they are needed for correcting field observations to a common hour for secular‐variation discussions. By comparing the annual mean diurnal variation in inclination at the United States Coast and Geodetic Survey observatories with the field results here discussed, three zones for the American continents are shown within which the phase of the curves is reversed; because of the shifting of the times of maximum and minimum with the season, the central lines of these zones are only approximately determined, but they may be placed at about magnetic latitude 40° north, 0°, and 25° south.