Low‐latitude ionospheric D region dependence on solar zenith angle

Abstract

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 ± 0.3 km and β = 0.49 ± 0.02 km À1 for SZA~10°, at midday, to H′ > 80 km and β~0.30 km À1as the SZA approached~70°-90°, near dawn and dusk for this tropical path. Additional values for the variations of H′ and β 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 β were found between low and middle latitudes resulting from the latitudinally varying interplay between Lyman α and galactic cosmic rays in forming the lower D region. Both latitude ranges showed β < 0.30 km À1during sunrise/sunset conditions.

Midlatitude ionospheric D region: Height, sharpness, and solar zenith angle

“…Because the galactic cosmic ray intensity at ionospheric heights varies with geomagnetic shielding and hence geomagnetic latitude [e.g., Heaps , ], the variations with SZA in H ′ and β are dependent on geomagnetic latitude as has been observed and reported by Thomson et al . []. The highest‐latitude path reported there was for NAU in Puerto Rico to St. John's in Canada for which the path midpoint is at a geographic latitude of 33° and a geomagnetic dip latitude of 37° as compared with the much higher values of 54.5° and 52.5°, respectively, for the current path.…”

“…From Thomson et al . [], when the mean SZA on the NAU to St. John's path changed from 33° to 70°, H ′ changed from 71.6 km to 76.9 km and β from 0.41 km −1 to 0.285 km −1 , while from Figure , for the DHO‐Eskdalemuir path, for the same SZA change from 33° to 70°, H ′ changed from 72.8 km to 76.2 km and β from 0.345 km −1 to 0.275 km −1 . The lower β and the smaller changes in β and H ′ with SZA on the DHO‐Eskdalemuir path are likely due to the greater proportion of non‐SZA‐dependent ionization (galactic cosmic rays) on this higher‐latitude path.…”

D‐Region High‐Latitude Forcing Factors

“…One key point in the data processing of atmospheric parameters is the separation between daytime and nighttime mesospheric conditions. Since the reflection height of the VLF waves changes between about 70 km during the day to 90 km at night (Thomson et al, , , ), we separated the temperature profiles into daytime and nighttime height ranges which were set as 70–75 and 83–88 km, respectively. The same criteria is not applied to the NO profiles because of the large amounts of missing data in those range of altitudes and so, due to this limitation, NO VMR data in the height range of 90–95 km were used for both daytime and nighttime analyses.…”

Perturbations to the lower ionosphere by tropical cyclone Evan in the South Pacific Region

“…In this model, the electron density N e as a function of height h is given as follows: where H′ is in km and β in km −1 . This formulation is commonly used in research using the technique of VLF signals [e.g., Thomson and McRae , ; Thomson et al , ]. With the geographic location of the transmitter and the receiver and the time in UTC as inputs, the code determines a single ambient value of each of the two parameters which is used for the complete transmitter‐receiver link [ Poulsen et al , ].…”