Observations and analysis of geomagnetic field variations near the magnetic equator

Abstract

In 1961 a magnetic observatory was established at Freetown, Sierra Leone (13º 13'W, 8º 28'N) just north of the dip equator. Records for H, D and Z for the period July 1961 to June 1965, covering the recent minimum in sunspot activity, have been analysed by the method due to Chapman and Miller to give the first four solar and luni-solar harmonics of the daily variation. The data was divided into three seasonal sets and two groups, firstly the International Quiet Days and secondly all days having magnetic activity index Ci ≤ 1.2. Probable errors for all harmonics have been determined and plotted on the harmonic dials. The seasonal changes of the solar terms show a movement of the equatorial current system in opposition to the sun with a larger shift during the northern summer months than during the northern winter months. The seasonal variations of the lunar terms show similar changes indicating that the lunar ionospheric current system behaves in a similar way to the solar current system. The occurrence of pulsations in the horizontal intensity of period approximately two minutes has been analysed for the year from March 1962 to February 1963. Two maxima were found, one at dawn and the second at noon. The dawn maxima was absent during northern winter months. A short field survey was undertaken to enable the plotting of magnetic charts for Sierra Leone. Measurements were also made of the daily variations at three field stations in Sierra Leone which confirmed the day-to-day variability of the daily variations of the equatorial ionospheric current system.