RADIO FADING AND THE NATURAL ELECTROMAGNETIC BACKGROUND

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Abstract
A comparison is made between the natural geomagnetic background in the frequency range 0.006–0.6 Hz and fluctuations of the difference in received signal strength between the two magnetoionic modes of vertically incident broad-band radio signals in the range 6–16 MHz, observed at a single station near Victoria, British Columbia. At this station the following points were observed:1. During the daytime there is often a spectral component common to the radio-signal fluctuations and to the natural geomagnetic background, even during quiet magnetic times. There is seldom phase coherence, however, except for a few minutes at a time.2. There is a small but highly significant (1%) linear correlation R = 0.33 between all the hourly mean amplitudes of the radio-signal fluctuation level and the hourly mean amplitudes of the natural geomagnetic background. The correlation coefficient rises as high as R = 0.71 at 1400–1500 local time, when only a particular hour of each day is considered. It is concluded that geomagnetic micropulsations in this frequency range are a better indicator of r-f. fluctuations than is the local Kp.3. At night, there is a relationship between the occurrence of Pt's and of r-f. fluctuation, although their frequency components are not usually as closely related as are those of the daytime regimes.It is suggested that geomagnetic micropulsations can couple exospheric or auroral zone fluctuations into the upper E or F regions strongly enough to produce observable radio-signal fluctuations at mid-latitudes, even during magnetically quiet times.Lack of correlation between micropulsations and phase fluctuations of 18 kHz signals observed over an E–W 100-km path, and 80 kHz observed over an E–W 3 300-km path suggests that significant coupling does not extend down to the D region during magnetically quiet times.

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