INTERPRETATION OF PRE-SUNRISE ELECTRON DENSITIES AND NEGATIVE IONS IN D-REGION

Pre-sunrise rocket measurements of the electron density in the D-region were carried out at Wallops Island, Virginai, on 15 July 1964, 14 June 1965, and 14 June 1966, when the solar zenith angle was near 95°; these rocket measurements were complemented by ground-based, steep-incidence VLF amplitude measurements of the NSS skywave. On 15 July 1964 and 14 June 1965, the rocket measurements revealed the presence of electrons in the D-region when the solar zenith angle was near 95°; and the VLF recordings showed absorption commencing at χ = 94°, approximately. However, during the pre-sunrise period on 14 June 1966, the VLF absorption commenced when the solar zenith angle was near 98°; and a rocket experiment near χ = 94° revealed an electron density profile almost identical to the one obtained on 14 June 1965, the exception being a ledge of ionization between 85 and 90 km. Based on the rocket electron density and VLF measurements, and assumptions of the height distribution of ozone, it is suggested that the VLF amplitude decreases near χ = 94° and χ = 98° are caused by the photodetachment of electrons from high-electron-affinity negative ions by solar u.v. radiation. Also, it is believed that an occasional VLF amplitude decrease commencing near χ = 99° is caused by the photodetachment of electrons from a low-electron-affinity negative ion by solar visible radiation. © 1968.