Further Observations of Sunrise and Sunset Fading of Very‐Low‐Frequency Signals

Abstract

An earlier expl~nation of sunrise and sunset fading on long VLF paths is confirmed by examining data t~ken over a ~1der range of frequency and directions of propagation. It is found. however. that _there IS an app~ec1able dependence of the fadinl! period on the magnetic direction of propagation. It IS shown that this dependence is qualitatively in agreement with what would be expected from a sharply bounded ionosphere and a transverse magnetic field.

Propagation of VLF Waves Below Anisotropic Ionosphere Models With a Dipping Static Magnetic Field

“…The interference distance D of the two lowest waveguide modes is increased for propagation in the east-to-west (EW) direction relative to the west· to-east (WE) direction for frequencies less than 25 kHz and for the angles between the vertical and the direction of the static magnetic field in the range of 20" to 60". These results are consistent with the recently reported measurements by Crombie (1966). The attenuation rates computed for the sharply bounded ionosphere model are in excess of 5 dB/1000 km for EW direction of propagation, but are lower in the stratified ionosphere models.…”

“…However, less successful have been the attempts for explaining differential phase velocity measurements of the two lower waveguide modes. Crombie (1966) has shown that in the frequency range 18 to 24kHz the observed interference distanceD= A. 1 A.…”

Frequency Dependence of VLF Fading at Sunrise

“…In earlier work (Crombie, 1964(Crombie, , 1966 on the analysis of the sunrise fading of VLF signals it was demon· strated that the distance D moved by the sunrise terminato-r" between successive signal minima was related to the wavelengths A. 1 , A2 of the first-and second-order modes in the nighttime portion of the path, and was expressed by This analysis can be extended to show that the difference in attenuation of the two modes in the nighttime portion of the path can be determined from changes in the successive maximum deviation (<P) of the resultant phase from the expected single mode phase and from the distance (S) which the terminator moves between the successive measurements of <P. Although the above work seemed to explain the sunrise variations in 18-to 24-kHz signals which propagated over paths that varied in length and direction, further confirmation was desirable.…”

Some studies on the stepped fall of atmospherics during sunrise

“…The stepwise amplitude minima with simultaneous phase advance was first investigated by Rieker [ 1 1 ] who explained it by a stepwise shortening of the lowest order skywave path during sunrise. The periodic field strength variation during sunrise on VLF signals propagated over long distances were investigated by Crombie [5,6] and Ries [10] who explained the steps by the waveguide mode theory. In the case of VLF atmospherics, however, radiation from distant sources would be masked by the stronger fields due to nearer sources.…”