Faraday rotation measurements of electron content near the magnetic equator

At the Electronics Laboratory of the Military Research and Development Center, Bangkok, Thailand (13.7°N latitude, 100.1°E longitude), Faraday rotation observations of the 54-MHz signal from the Transit 4-A (1961 Omicron 1) satellite were made during 10 months of a sunspot minimum period (1964). The satellite has an essentially circular orbit with an inclination of 67° and a spin-stabilized linear-polarized antenna. In this paper, two methods of analysis, involving rotation rate and total number of rotations, are applied to the Bangkok observations. The rotation-rate method is used to determine the electron content when the angle between the ray path and the geomagnetic field is 90° (transverse position). The total-rotations method is used to determine the latitudinal variations of the electron content for five selected satellite passes from a joint analysis of observations at three stations, Singapore, Bangkok, and Hong Kong. A comparison of the calculated critical frequency with observed ionosonde critical frequencies at Singapore, Bangkok, and Macao shows some evidence that the equivalent slab thickness may be considered independent of latitude near the magnetic equator.