A whistler is a very low frequency electromagnetic (radio) wave which can be generated, for example, by lightning. Frequencies of terrestrial whistlers are 1 to 30 kHz, with maximum usually at 3 to 5 kHz. Although they are electromagnetic waves, they occur at audio frequencies, and can be converted to audio using a suitable receiver. They are produced by lightning strikes (mostly intracloud and return-path) where the impulse travels away from the earth and returns to the earth traveling along magnetic field lines. .The study of whistlers allows categorization into Pure Note Whistlers, Diffuse, 2-hop, and Echo Train types. They undergo dispersion of several thousand kHz due to the slower velocity of the lower frequencies through the plasma environments of the ionosphere and magnetosphere. Thus they are perceived as a descending tone which can last for a few seconds. The study of whistlers allows categorization into Pure Note Whistlers, Diffuse, 2-hop, and Echo Train types.
In addition, Voyager 1 and 2 spacecraft detected whistler-like activity in the vicinity of Jupiter, implying the presence of lightning there.
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The existence of whistlers was first noticed by Barkhausen in 1918. In 1953, Storey explained the origin of them, showing that whistlers are originated by lightning discharges.
The propagation of a signal in the Earth Ionosphere wave guide is known as a sferic. If the signal is injected through the ionosphere and allowed to propagate through the magnetosphere, the signal is now prone to a bounce-mode propagation, reflecting back and forth on opposite sides of the planet until totally attenuated. To clarify which part of this hop pattern the signal is in, it is specified by a number, indicating the portion of the bounce path it is currently on.[1] On its first upward path, it is known as a 0+. After passing the geomagnetic equator, it is referred to as a 1-. The + or - sign indicates either upward, or downward, propagation, respectively. The numeral represents the half-bounce currently in progress. The reflected signal is redesignated 1+, until passing the geomagnetic equator again; then it is called 2-, and so on.
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