Daytime D region parameters from long-path VLF phase and amplitude

Observed phases and amplitudes of VLF radio signals propagating on very long paths are used to validate electron density parameters for the lowest edge of the (D region of the) Earth's ionosphere at low latitudes and midlatitudes near solar minimum. The phases, relative to GPS 1 s pulses, and the amplitudes were measured near the transmitters (∼100–150 km away), where the direct ground wave is dominant, and also at distances of ∼8–14 Mm away, over mainly all-sea paths. Four paths were used: NWC (19.8 kHz, North West Cape, Australia) to Seattle (∼14 Mm) and Hawaii (∼10 Mm), NPM (21.4 kHz, Hawaii) and NLK (24.8 kHz, Seattle) to Dunedin, New Zealand (∼8 Mm and ∼12 Mm). The characteristics of the bottom edge of the daytime ionosphere on these long paths were found to confirm and contextualize recently measured short-path values of Wait's traditional height and sharpness parameters, H′ and β, respectively, after adjusting appropriately for the (small) variations of H′ and β along the paths that are due to (1) changing solar zenith angles, (2) increasing cosmic ray fluxes with latitude, and (3) latitudinal and seasonal changes in neutral atmospheric densities from the (NASA) Mass Spectrometer Incoherent Scatter- (MSIS-) E-90 neutral atmosphere model. The sensitivity of this long-path (and hence near-global) phase and amplitude technique is ∼ ± 0.3 km for H′ and ∼ ± 0.01 km−1 for β, thus creating the possibility of treating the height (H′ ∼70 km) as a fiduciary mark (for a specified neutral density) in the Earth's atmosphere for monitoring integrated long-term (climate) changes below ∼70 km altitude.