Experimental measurements of ambient electromagnetic noise from 1.0 to 4.0 kHz

The spectral noise density, amplitude probability distribution (APD), and spectral content of ambient noise from 1.0 to 4.0 kHz were measured. Analysis of the recorded data included detailed investigation of the diurnal variation of the noise, comparison of the APDs with two atmospheric noise models, time domain excision of noise impulses to assess the reduction in noise achievable by nonlinear processing, investigation of broadband ionospheric emissions identified as polar chorus, and determination of the statistical variation (in frequency) of the spectral minimum near 3 kHz. The analysis of the diurnal variation suggested the existence of a large propagation anisotropy in the nighttime Earth ionosphere waveguide at 3 kHz, with east to west attenuation significantly lower than north-south attentuation. The observed seasonal and geographical averaged diurnal variations can be interpreted qualitatively in terms of worldwide thunderstorm distributions and day-night changes in the reflecting height of the ionospheric D-layer. The measured spectral noise density values are significantly lower than values measured under similar conditions by earlier investigations; the lower values are attributed to the use of a receiver with a lower intrinsic noise and careful site selection to avoid man made interference. The recorded APDs varied from a Rayleigh form during low noise periods in Norway to a distribution with a tail at low exceedence values during higher noise periods in both Norway and Italy. Comparison of the APDs with the Field-Lewenstein model and the Middleton model generally showed satisfactory agreement.