Low-latitude ionospheric D region dependence on solar zenith angle

Phase and amplitude measurements of VLF radio signals on a short, nearly all-sea path between two Hawaiian Islands are used to find the height and sharpness of the lower edge of the daytime tropical D region as a function of solar zenith angle (SZA). The path used was from U. S. Navy transmitter NPM (21.4 kHz) on Oahu to Keauhou, 306 km away, on the west coast of the Big Island of Hawaii, where ionospheric sensitivity was high due to the destructive interference between the ionospherically reflected wave and the ground wave, particularly around the middle of the day. The height and sharpness are thus found to vary from H'=69.3 +/- 0.3km and beta=0.49 +/- 0.02 km(-1) for SZA similar to 10 degrees, at midday, to H'>80 km and beta similar to 0.30 km(-1) as the SZA approached similar to 70 degrees-90 degrees, near dawn and dusk for this tropical path. Additional values for the variations of H' and beta with solar zenith angle are also found from VLF phase and amplitude observations on other similar paths: the short path, NWC to Karratha (in NW Australia), and the long paths, NWC to Kyoto in Japan and NAU, Puerto Rico, to St. John's Canada. Significant differences in the SZA variations of H' and beta were found between low and middle latitudes resulting from the latitudinally varying interplay between Lyman a and galactic cosmic rays in forming the lower D region. Both latitude ranges showed beta<0.30 km(-1) during sunrise/sunset conditions.