A comparison is made between the phase and amplitude of the 18‐kc/s signal NBA (Balboa, Panama) recorded at Boulder, Colorado, in the northern hemisphere and Tucumán, Argentina, in the southern hemisphere. Although these two propagation paths are essentially the same length, the difference between them in the diurnal change in phase height is approximately 5 km, and the estimated field strength appears to be significantly lower at Tucumán than at Boulder. It is suggested that the cause of these observed differences is the latitudinal variation in ionization due to cosmic rays.
VLF amplitude data taken at frequencies from 9.2 to 17.2 kHz on two long paths are analyzed in terms of diurnal transmission loss differences. The observed values are in fair accord with theoretical values for the first mode, and the effect of the second mode could account for the observed data scatter in the frequency range of the calculations.
Recent nighttime phase and amplitude anomalies have been observed on the 10.2 and 13.6 kHz signals from Trinidad, West Indies, as received at Boulder, Colorado. The anomalies, in some cases, are comparable in size to the normal diurnal variations of these signals and occur predominantly at 13.6 kHz. These observations, when taken with independent confirming observations, suggest localized changes in the ionosphere or in VLF mode structure that may be significant to radio navigation.
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