Measured Statistical Characteristics of VLF Atmospheric Radio Noise

Instrumentation for measuring the cumulative distribution of the amplitudes and spacings of pulses in the instantaneous envelope of the atmospheric noise field strength is described. In general, the vlf atmospheric noise observed at 22 kc in a 1-kc band during the fall of 1955 from 9°N to 71°N latitude was found to have a maximum variation in average power level, including the effects of both time and geographic location, of about 46 db. The dynamic range of the instantaneous noise envelope, measured during a 20- to 30-minute period of time, is defined to be the ratio of the field strength exceeded 0.0001 per cent of the time to that exceeded 90 per cent of such periods of time. This dynamic range in a l-kc band, for the 66 periods measured, varied from 59 to 102 db. The average dynamic range in the Arctic was 68 db and in the tropics 81 db. The noise envelope at the low amplitude levels is found to be Rayleigh distributed, while that at the higher levels approaches a distribution having a much greater change in level for a given change in probability. In general, at higher levels, the spacing between pulses does not appear to be random at temperate and arctic locations, but the noise pulses observed in the tropics appear to be more randomly spaced. When the bandwidth of the receiver is reduced, the dynamic range approaches 21.18 db, the value expected for the Rayleigh distributed envelope resulting from a thermal noise input.