In recent years, measurements by rocket, balloon, and satellite of cosmic X rays have suggested the possibility of observing ionospheric effects from celestial X-ray sources such as Scorpius XR-1, using low and very low frequency radio transmissions. The behavior of the nighttime D region under ionization by the X-ray spectrum of the Scorpius source is estimated for several stellar zenith angles. Although Scorpius XR-1 is the strongest source in the southern hemisphere, the integrated flux of all the other X-ray sources is comparable to that of Scorpius XR-1 and appears to modify the effect of Scorpius XR-1 alone on VLF data recorded in the southern hemisphere. Observations of the VLF transmission NBA (18 kHz, Balboa, Panama) made at Tucuman, Argentina, during 1963–1964 show a sidereal time variation in the diurnal amplitude change that follows the variation in the per cent illumination of the propagation path by Scorpius XR-1 during the nighttime hours. Even though the diurnal amplitude change can be shown to have a sidereal time variation, the diurnal phase change at Tucuman, Argentina, does not vary much throughout the year and yet remains consistently lower than that observed over a path of the same length from NBA to Boulder, Colorado, in the northern hemisphere. Therefore, it is suggested that the integrated effect of all the X-ray sources illuminating the southern hemisphere is to enhance the ionization in the southern hemisphere nighttime D-region, thus, producing the smaller diurnal phase change observed at Tucuman compared with that observed on a Boulder recording of the NBA signal over a propagation path of the same length in the northern hemisphere.
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