A whistler is a very low frequency (VLF) electromagnetic (radio) wave created by lightning. Terrestrial whistlers have frequencies between 1 kHz and 30 kHz, with the most common frequencies between 3 kHz and 5 kHz. Although these are radio waves, they can be changed into sound using a special receiver. Lightning strikes, especially those between clouds or returning to the ground, produce whistlers. These waves travel along Earth's magnetic field lines from one hemisphere to the other. As they move through the ionosphere and magnetosphere, they spread out into different frequencies because lower frequencies move more slowly through plasma. This causes whistlers to sound like a fading note that lasts several seconds. Scientists classify whistlers into four types: Pure Note, Diffuse, 2-Hop, and Echo Train.
The Voyager 1 and 2 spacecraft found whistler-like activity near Jupiter, called "Jovian Whistlers," which matched observations of lightning made by Voyager 1.
Whistlers have also been detected in Earth's magnetosheath, where they are sometimes called "lion roars" because their frequencies range from tens to hundreds of Hz.
History
Whistlers were likely heard as early as 1886 on long telephone lines. However, the clearest early description of whistlers was provided by Heinrich Barkhausen in 1919. In 1953, British scientist Llewelyn Robert Owen Storey showed that lightning can create whistlers in his PhD dissertation. Around the same time, Storey suggested that whistlers could prove that plasma exists in Earth’s atmosphere and that this plasma moves radio waves in the same direction as Earth’s magnetic field lines. From this, Storey concluded that a thin layer called the plasmasphere exists between the ionosphere and magnetosphere, though he could not prove it for sure. In 1963, American scientist Don Carpenter and Soviet astronomer Konstantin Gringauz, working independently and using data from the Luna 2 spacecraft, experimentally confirmed the existence of the plasmasphere and plasmapause, building on Storey’s ideas.
American electrical engineer Robert Helliwell is also known for his research on whistlers. In 1950, Helliwell and his student, Jack Mallinckrodt, studied lightning noise at very low radio frequencies at Stanford University. Mallinckrodt noticed some whistling sounds and told Helliwell. As Helliwell later described in an article from 1982, he first thought the sounds were an error, but after listening with Mallinckrodt, he confirmed their presence. Helliwell described the sounds as "weird, strange, and unbelievable as flying saucers" in a 1954 article. Helliwell worked to understand how whistlers form. He conducted experiments at the VLF outpost Siple Station in West Antarctica, which operated from 1971 to 1988. Because very low frequency (VLF) radio signals have very long wavelengths (for example, a 10 kHz signal has a wavelength of 30 kilometers or 19 miles), Siple Station used an antenna 13 miles (21 kilometers) long. This antenna sent VLF signals into Earth’s magnetosphere, where they were detected in Canada. Scientists could inject these signals into the magnetosphere because the ionosphere allows these low frequencies to pass through.
The term "whistlers" was given by British World War I radio operators. On a wide-band spectrogram, whistlers are seen as tones that quickly drop in pitch over a few seconds—similar to a person whistling or a grenade approaching—hence the name "whistlers."
Nomenclature
A type of electromagnetic signal, called a radio atmospheric signal or sferic, can travel through the Earth–ionosphere waveguide. Sometimes, this signal escapes the ionosphere and moves into the magnetosphere. The signal often bounces back and forth, reflecting between opposite sides of the planet until it is completely weakened. To describe which part of this bouncing path the signal is on, it is given a number. This number shows the current portion of the bounce path. On its first upward journey, the signal is called a 0. After it crosses the geomagnetic equator, it is labeled a 1. The + or – sign shows whether the signal is moving upward or downward. The number represents the current half of the bounce path. When the signal reflects and moves again, it is relabeled as 1 until it crosses the geomagnetic equator once more. At that point, it is called 2, and this pattern continues.