Tropical daytime lower D-region dependence on sunspot number

Thomson, Neil R.; Rodger, Craig J.; Clilverd, Mark A. ORCID: 2012 Tropical daytime lower D-region dependence on sunspot number. Journal of Geophysical Research, 117 (A10). 9, pp.

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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.

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: BAS Programmes > Polar Science for Planet Earth (2009 - ) > Climate
ISSN: 0148-0227
Additional Keywords: Earth-ionosphere waveguide height, VLF propagation, Daytime D-region electron density, Height dependence on sunspot number,Ionosphere, Radio Science
Date made live: 14 Nov 2012 14:42 +0 (UTC)

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