Experimental Observation of Magnetic Field Effects on VLF Propagation at Night

Abstract

Computations of nighttime field intensity versus distance are made for a 23.4‐kHz signal radiated from Hawaii and for propagation paths to Seattle, Ontario (California), Samoa, and Wake Island. The computations were made by using the waveguide computer program developed at the Naval Electronics Laboratory Center to obtain waveguide mode constants, each 2° of arc or 222 km along each path. An exponential electron‐density profile defined by β = 0.5 km −1 and h ′ = 85.5 was assumed, where β and h ′ are defined by Wait [1964]. The resultant field was computed by using a WKB approximation to allow for the variation of mode constants along the paths. Experimental measurements of 23.4‐kHz signals from NPM were made aboard an airplane as it flew along these propagation paths. (NPM are the call letters for the Lualualei Navy Radio Station in Hawaii.) Good agreement was obtained, between the theoretical calculations and experimental measurements, strongly supporting the validity of the theoretical approach used (the β = 0.5, h ′ = 85.5 km profile assumed) and the conclusion that the increased attenuation observed for propagation to the south is an effect of the geomagnetic field.