Audio‐frequency fluctuations in the geomagnetic field

Audio-frequency geomagnetic fluctuation spectra are reported for three North American stations covering a latitude range of 41°. Recording stations were located at Panama City, Florida; White Oak, Maryland; and Point Barrow, Alaska. Fluctuation intensities throughout the frequency range 70 to 16,000 cps were observed to decrease with increasing latitude. Between 100 and 1000 cps, fluctuation amplitudes were inversely proportional to about the 3/2 power of frequency, the spectral slopes becoming somewhat steeper with increasing latitude. Between 1500 and 8000 cps, amplitudes decreased more rapidly with latitude than did those at other frequencies, and a spectral minimum was evident in this frequency range at Point Barrow. The vertical fluctuation component was smaller and decreased more rapidly with latitude than did the horizontal components. It is concluded that thunderstorm activity is the major source of the fluctuations throughout the audio-frequency range, with the magnetic atmospherics being generated chiefly in the lower latitudes and propagated poleward. The relationship of the observed fluctuations to thunderstorm distributions is discussed. There is considerable agreement with the results of others, although a few discrepancies are apparent.