On the Determination of VLF Propagation Parameters by Field Strength Measurements Over Medium Distances

Abstract

The propagation of electromagnetic waves with a frequency of 16 kc/s is studied in the distance range 350 to 800 km from the. transmitter GBR Rugby. Field strength measurements recorded contmuously at eight stations durmg the winter of 1962/63 are compared with field strength calculations based upon the concept of the equivalent ionosphere.The influence of the lower ionosphere on long electromagnetic waves for a specific frequency and propagation direction is nearly the same as that of a sharply bounded, homogeneously ionized isotropic equivalent ionosphere. For oblique incidence the boundary can be taken at a certain “equivalent height” h′, if the reflection coefficient is approximated by the function ‐ exp (‐ α cos ϑ), ϑ being the angie of incidence, and a a certain real positive parameter characterizing the reflection. The possible error caused by this approximation for the distance range mentioned above is considered. The two parameters h′ and a are determined from the measurements.The possible range of parameters for nighttime and daytime propagation is first estimated by different compansons between calculated and measured field strength curves. The sunrise effect is then comsidered and evidence is found that during about 1.5 hr after sunrise the concept of the equivalent ionosphere does not satisfactorily describe the propagation.A method is presented for finding the parameters for nighttime and daytime propagation using only the relative field strength variations as measured at the different distances.Typical fall daytime values for h′ and ϑ are near 69 km and 4, respectively; for winter days h′ is about 73 km and a is of the order of unity, both varying in rather wide limits. For the nighttime, h′ ‐values were found to be between 81 and 89 km and a was smaller than 0.3 in most cases, corresponding to a reflection of a strong dielectric type.