Tropical daytime lower D-region dependence on sunspot number

Observed phases and amplitudes of VLF radio signals propagating on (near) tropical all-sea paths, both short, ∼300 km, and long, ∼10 Mm, are used to find daytime parameter changes for the lowest edge of the (D-region of the) Earth's ionosphere as the solar cycle advanced from a very low sunspot number of ∼5 up to ∼60, in the period 2009–2011. The VLF phases, relative to GPS 1-s pulses, and amplitudes were measured ∼100 km from the transmitter, where the direct ground wave is very dominant, ∼300 km from the transmitter, near where the ionospherically reflected waves form a (modal) minimum with the ground wave, and ∼10 Mm away where the lowest order waveguide mode is fully dominant. Most of the signals came from the 19.8 kHz, 1-MW transmitter, NWC, North West Cape, Australia, propagating ENE, mainly over the sea, to the vicinity of Karratha and Dampier on the NW coast of Australia and then on to Kauai, Hawaii, ∼10.6 Mm from NWC. Observations from the 8.1-Mm path NPM (21.4 kHz, Hawaii) to Dunedin, NZ, are also used. The sunspot number increase from ∼5 to ∼60 was found to coincide with a decrease in the height, H′, of the midday tropical ionosphere by 0.75 ± 0.25 km (from H′ ≈ 70.5 km to H′ ≈ 69.7 km) while the sharpness, β increased by 0.025 ± 0.01 km−1 (from β ≈ 0.47 km−1 to β ≈ 0.49 km−1) where H′ and β are the traditional height and sharpness parameters used by Wait and by the U.S. Navy in their Earth-ionosphere VLF radio waveguide programs.