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Concept
Whistler (radio)
Summary
A whistler is a very low frequency (VLF) electromagnetic (radio) wave generated by lightning. Frequencies of terrestrial whistlers are 1 kHz to 30 kHz, with maximum frequencies usually at 3 kHz 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 along the Earth's magnetic field lines from one hemisphere to the other. They undergo dispersion of several 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 categorizes them into Pure Note, Diffuse, 2-Hop, and Echo Train types.
Voyager 1 and 2 spacecraft detected whistler-like activity in the vicinity of Jupiter known as "Jovian Whistlers", supporting the visual observations of lightning made by Voyager 1.
Whistlers have been detected in the Earth's magnetosheath, where they are often called “lion roars” due to their frequencies of tens to hundreds of Hz.
The pulse of electromagnetic energy of a lightning discharge producing whistlers contains a wide range of frequencies below the electron cyclotron frequency. Due to interactions with free electrons in the ionosphere, the waves becomes highly dispersive and like guided waves, follow the lines of geomagnetic field. These lines provide the field with sufficient focusing influence and prevents the scattering of field energy. Their paths reach into the outer space as far as 3 to 4 times the Earth's radius in the plane of equator and bring energy from lightning discharge to the Earth at a point in the opposite hemisphere which is the magnetic conjugate of the position of radio emission for whistlers. From there, the whistler waves are reflected back to the hemisphere from which they started. The energy is almost perfectly reflected from earth surface 4 or 5 times with increase dispersion and diminishing amplitude.
Official source
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