Whistler radio palmer 2004-07-23 T035528

A whistler is a

very low frequency Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave a ...

(VLF) electromagnetic (radio) wave generated by

lightning Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an average ...

. Originally published by Stanford University Press, Stanford, California (1965). Frequencies of terrestrial whistlers are 1 kHz to 30 kHz, with a maximum

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...

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 strike A lightning strike or lightning bolt is an electric discharge between the atmosphere and the ground. Most originate in a cumulonimbus cloud and terminate on the ground, called cloud-to-ground (CG) lightning. A less common type of strike, ground- ...

s (mostly intracloud and return-path) where the impulse travels along the

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...

lines from one hemisphere to the other. They undergo

dispersion Dispersion may refer to: Economics and finance *Dispersion (finance), a measure for the statistical distribution of portfolio returns *Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variatio ...

of several kHz due to the slower velocity of the lower frequencies through the plasma environments of the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

and

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior d ...

. 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 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin '' Voyager 2'', ''V ...

and 2 spacecraft detected whistler-like activity in the vicinity of

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...

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.

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.

History

Whistlers were probably heard as early as 1886 on long telephone lines, but the clearest early description was by

Heinrich Barkhausen Heinrich Georg Barkhausen (2 December 1881 – 20 February 1956), born in Bremen, was a German physicist. Growing up in a patrician Bremen family, he showed interest in natural sciences from an early age. He studied at the Technical Univers ...

in 1919. British scientist Llewelyn Robert Owen Storey had shown

generated whistlers in his 1953 PhD dissertation. Around the same time, Storey had posited the existence of whistlers meant plasma was present in

Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...

, and that it moved radio waves in the same direction as

lines. From this he deduced but was unable to conclusively prove the existence of the plasmasphere, a thin layer between the

and

. In 1963 American scientist

Don Carpenter Don Carpenter (March 16, 1931 – July 27, 1995) was an American writer, best known as the author of '' Hard Rain Falling''. He wrote numerous novels, novellas, short stories and screenplays over the course of a 22-year career that took him from ...

and Soviet

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either ...

Konstantin Gringauz The first name Konstantin () is a derivation from the Latin name ''Constantinus'' ( Constantine) in some European languages, such as Russian and German. As a Christian given name, it refers to the memory of the Roman emperor Constantine the Gr ...

—independently of each other, and the latter using data from the ''

Luna 2 ''Luna 2'' ( rus, Луна 2}), originally named the Second Soviet Cosmic Rocket and nicknamed Lunik 2 in contemporaneous media, was the sixth of the Soviet Union's Luna programme spacecraft launched to the Moon, E-1 No.7. It was the first spa ...

'' spacecraft—experimentally proved the plasmasphere and plasmapause's existence, building on Storey's thinking. 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 Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...

at very low radio frequencies at

Stanford University Stanford University, officially Leland Stanford Junior University, is a private research university in Stanford, California. The campus occupies , among the largest in the United States, and enrolls over 17,000 students. Stanford is conside ...

in 1950. Mallinckrodt heard some whistling sounds and brought them to Helliwell's attention.Melissae Fellet, "Robert Helliwell, Radioscience and Magnetosphere Expert, Dead at 90," Stanford Report, May 20, 2011 at http://news.stanford.edu/news/2011/may/robert-helliwell-obit-052011.html 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''. Helliwell tried to understand the mechanism involved in the production of whistlers. He conducted experiments at the VLF outpost Siple Station in

West Antarctica West Antarctica, or Lesser Antarctica, one of the two major regions of Antarctica, is the part of that continent that lies within the Western Hemisphere, and includes the Antarctic Peninsula. It is separated from East Antarctica by the Transan ...

, which was active from 1971 to 1988. Since the

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

of VLF radio signals is very large (a

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

of 10 kHz corresponds to a wavelength of ), Siple Station had an

antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...

that was long. The antenna was used to transmit VLF radio signals into Earth's magnetosphere, to be detected in

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...

. It was possible to inject these signals into the magnetosphere, since the ionosphere is transparent to these low frequencies.

Etymology

Whistlers were named by British

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...

radio operators. 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 Whistling without the use of an artificial whistle is achieved by creating a small opening with one's lips, usually after applying moisture (licking one's lips or placing water upon them) and then blowing or sucking air through the space. The a ...

or an incoming

grenade A grenade is an explosive weapon typically thrown by hand (also called hand grenade), but can also refer to a shell (explosive projectile) shot from the muzzle of a rifle (as a rifle grenade) or a grenade launcher. A modern hand grenade ge ...

—hence the name "whistlers."

Nomenclature

A type of electromagnetic signal propagating in the

Earth–ionosphere waveguide The Earth–ionosphere waveguide refers to the phenomenon in which certain radio waves can propagate in the space between the ground and the boundary of the ionosphere. Because the ionosphere contains charged particles, it can behave as a conduct ...

, 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. On its first upward path, it is known as a 0⁺. After passing the

geomagnetic equator Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the Earth's magnetic field lines. This angle varies at different points on the Earth's surface. Positive values of inclination indicate that the magnetic fi ...

, 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.

Sources

History

Etymology

Nomenclature

See also

Relevant spacecraft

References

Further reading