Solar cycle changes in daytime VLF subionospheric attenuation

Observations of field strengths of signals from a number of VLF transmitters, after propagation over long paths in the Earth–ionosphere waveguide, have been used to examine changes in the daytime attenuation rate in the course of a solar cycle. The measurements reported were recorded in the period 1986–1996. The paths studied range in length over about 8–14 Mm; they included NLK (Seattle) and NPM (Hawaii) to Dunedin, NZ, and NAA (Cutler, Maine) and NSS (Annapolis, Maryland) to Faraday, Antarctica. The frequencies monitored were mainly in the range 21–25 kHz but measurements near 10 kHz of Omega Hawaii at Faraday were also used. The daytime VLF attenuation rates at solar minimum were found to be greater by about 0.3 dB/Mm than at solar maximum.