Atmospheric radio noise at frequencies between 10 kc/s and 30 kc/s

Measurements of the characteristics of very-low-frequency atmospheric noise in Southern England have been made with automatic equipment during the last few years. The results are described in terms of statistical parameters of the envelope at the output of a narrow-bandwidth receiver (300 c/s between 3 dB points). The average voltage of the envelope, recorded via an integrating circuit with equal charge and discharge time-constants of 8 sec, showed short-term fluctuations of about 5% and clear diurnal and seasonal variations. The r.m.s. voltage, deduced by integration of measured amplitude probability distributions, varied between 4 and 8 times the average voltage; the values were very dependent on the incidence of large infrequent pulses. The noise was always much more impulsive than fluctuation noise. The structure of the envelope was described by two forms of distribution, one being the amplitude distribution of the peaks of pulses, and the other the conventional amplitude probability distribution of voltage. The former was represented by a simple power-law function of voltage involving two parameters, the average voltage and the power-law index. The latter was represented by another simple function involving two similar parameters. The character of the noise was always approximately the same in spite of substantial changes of average voltage from time to time. Variations of the intensity and structure with frequency and band width were examined.