Further examples of seasonal variations of ELF radio propagation parameters

In this paper we use experimentally determined values of effective attenuat ion rate, excitation factor, and relative phase velocity, along with the th eoretical expressions derived by C. and P. Greifinger, to establish the sea sonal variation of representative ionospheric conductivity parameters. Thes e parameters include the reflection heights h(0) and h(1) (or h(E)), invers e scale height beta, and reference height H. The basis for this analysis is provided by the 1990-1992 76-Hz field strength measurements taken at four land-based ELF monitoring sites established by the U.S. Navy. The source fo r these measurements was the U.S. Navy's dual-antenna transmitting system ( WTF/MTF). The main conclusion of this paper is that the summertime and Janu ary nighttime attenuation rates are substantially lower than during other t imes of the year. This nighttime attenuation rate decrease appears to be ma inly due to an increase in the inverse scale height beta, rather than to an increase in the reflection heights h(0) and h(E).