A whistler is a very low frequency (VLF) electromagnetic (radio) wave generated by lightning.[1] 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",[2] supporting the visual observations of lightning made by Voyager 1.[3]
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.[4]
Sources
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. Along such long paths the speed of propagation of energy is between c/10 to c/100 (where c is the speed of light) and the exact value depends upon frequency.
Modulated heating of the lower ionosphere with an HF heater array can also be used to generate VLF waves that excite whistler mode propagation. By transmitting high power HF waves with a VLF modulated power envelope into the D-region ionosphere, the conductivity of the ionospheric plasma can be modulated. This conductivity modulation together with naturally occurring electrojet fields produces a virtual antenna which radiates at the modulation frequency. The HAARP HF heater array has been used to excite whistler-mode VLF signals detectable at the magnetic conjugate point, with up to 10 hops visible in the received VLF data. [5]
American electrical engineer Robert Helliwell is also known for his research into whistlers. Helliwell and one of his students, Jack Mallinckrodt, were investigating lightning noise at very low radio frequencies at Stanford University in 1950. Mallinckrodt heard some whistling sounds and brought them to Helliwell's attention.[8] As Helliwell recalled in an article in the October 1982 issue of the Stanford Engineer, he initially thought it was an artifact, but stood radio watch with Mallinckrodt until he heard the whistlers himself. Helliwell described these sounds as "weird, strange and unbelievable as flying saucers" in a 1954 article in the Palo Alto Times.[8] Helliwell tried to understand the mechanism involved in the production of whistlers. He conducted experiments at the VLF outpost Siple Station in West Antarctica, which was active from 1971 to 1988.[8] Since the wavelength of VLF radio signals is very large (a frequency of 10 kHz corresponds to a wavelength of 30 kilometres (19 mi)), Siple Station had an antenna that was 13 miles (21 km) long. The antenna was used to transmit VLF radio signals into Earth's magnetosphere, to be detected in Canada. It was possible to inject these signals into the magnetosphere, since the ionosphere is transparent to these low frequencies.[8]
Etymology
Whistlers were named by British World War I radio operators.[6] On the wide-band spectrogram, the observed characteristic of a whistler is that the tone rapidly descends over a few seconds—almost like a person whistling or an incoming grenade—hence the name "whistlers."[6]
Nomenclature
A type of electromagnetic signal propagating in the Earth–ionosphere waveguide, known as a radio atmospheric signal or sferic, may escape the ionosphere and propagate outward into the magnetosphere. The signal is prone to 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.[9] 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.
Helliwell, Robert A. (1958). "Whistlers and VLF Emissions". In Odishaw, Hugh; Ruttenberg, Stanley (eds.). Geophysics and the IGY: proceedings of the symposium at the opening of the International Geophysical Year. pp. 35–44.
