VLF modal interference over west-east paths

The contribution of cosmic X rays in the production of ionization in the nighttime D region is discussed in the light of observed X-ray flux emissions from the various celestial X-ray sources and from the galactic background. The cumulative effect of several sources when they are in transit at close time intervals can amount to a sizable fraction of all nighttime sources, which consist principally of scattered Ly α and Ly β. Above 95 km, the Ly β contribution is generally far in excess of the production due to cosmic X rays, but, at and around 90 km, the relative importances of cosmic X rays and scattered Ly α depend on the variation of the scattered Ly α flux, its variations with time, the nighttime concentrations of nitric oxide, and on the variations in the flux from the sources. Since some of these are uncertain, exact contributions are difficult to establish, but a marginal ionospheric effect around 90 km levels from cosmic X-ray sources seems possible at low latitudes.

Ionization Contribution by Cosmic X Rays

Some areas of D and E region research are examined, with particular reference to the photochemistry and transport of minor neutral constitutents in the mesosphere and lower thermosphere. The review shows that the D and E regions become more complex physical and chemical systems as research progresses, so that some previous views on these regions have to be revised. In particular, the lower ionosphere appears to be hydrodynamically coupled to the lower atmosphere and electrodynamically coupled to the magnetosphere.

Ionospheric D and E regions

The effect of short bursts of cosmic X- and γ-rays on the ionosphere

Comments on the ionospheric detection of cosmic X-ray phenomena

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.

C. J. Chilton

Papers

VLF Observations of Nighttime D‐Region Ionization Enhancement by the Scorpius XR‐1 X‐ray Source