Tropical daytime lower D-region dependence on sunspot number

摘要

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, increased by 0.025±0.01 km ~(-1)(fromβ≈ 0.47 km ~(-1)to fromβ≈0.49 km ~(-1))where H and are the traditional height and sharpness parameters used by Wait and by the U.S. Navy in their Earth-ionosphere VLF radio waveguide programs.