From the earliest days of radio experimentation, various kinds of very‐low‐frequency electromagnetic radiation have intrigued those who heard the strange mixture of glissandi whistles, hisses, chirps and warbling sounds on their headphones. One common type sounds like birds awakening in the morning and was appropriately named the “dawn chorus.” Confined usually to an audiofrequency range of 200 to 30 000 Hz, these emissions appeared to be distinct from other forms of “static”—locally initiated noise due to nearby lightning strokes, precipitation, or man‐made electrical interference. The observational equipment required for the study of such emissions is comparatively simple—a single‐turn loop antenna connected to a high‐gain, lownoise, wide‐band audio amplifier. If the output is recorded on magnetic tape, spectrograms can be produced by standard techniques. Time markers can be added either from a local clock or from a radio station.
VLF emissions from the magnetosphere
Ravindra Nath Sudan, Jacques Denavit; VLF emissions from the magnetosphere. Physics Today 1 December 1973; 26 (12): 32–39. https://doi.org/10.1063/1.3128360
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