Whistler (radio)

VLF spectrogram of an electromagnetic whistler wave, as received by the Stanford University VLF group's wave receiver at Palmer Station, Antarctica.

A whistler is a very low frequency electromagnetic (radio) wave which can be generated, for example, by lightning. Frequencies of terrestrial whistlers are 1 kHz to 30 kHz, with a maximum amplitude 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 away from the earth and returns to the earth traveling along magnetic field lines. 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, implying the presence of lightning there.

History

The existence of whistlers was first noticed by Barkhausen in 1918. In 1953, Storey showed that whistlers originate from lightning discharges.

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 now 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, 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.

See also

Related Topics

• Dawn chorus (electromagnetic)
• Electromagnetic electron wave
• Hiss (electromagnetic)
• Atmospheric noise
• Radio atmospheric

Relevant Spacecraft

• Advanced Composition Explorer (ACE), launched 1997, still operational.
• Helios (spacecraft)
• MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging), launched 2004, still operational.
• Radiation Belt Storm Probes
• Solar Dynamics Observatory (SDO), launched 2010, still operational.
• Solar and Heliospheric Observatory (SOHO), launched 1995, still operational.
• Solar Maximum Mission (SMM), launched 1980, decommissioned 1989.
• Solar Orbiter (SOLO), set to launch in 2015.
• Solar Probe Plus, set to launch in 2015.
• STEREO (Solar TErrestrial RElations Observatory), launched 2006, still operational.
• Transition Region and Coronal Explorer (TRACE), launched 1998, decommissioned 2010.
• Ulysses (spacecraft), launched 1990, decommissioned 2009.
• WIND (spacecraft), launched 1994, still operational.

Further reading

• A site containing recorded audio of natural VLF phenomena.
• A beginner's guide to natural VLF radio phenomena - second part.
• Natural VLF Radio - Whistlers.

• Robert A. Helliwell (2006 [1965]). Whistlers and Related Ionospheric Phenomena. Dover Publications, Inc. ISBN 0-486-44572-0.  Originally published by Stanford University Press, Stanford, California (1965). See also Whistlers and VLF Emissions by the same author. (These are best sought in a library)

• Romero, R. (2008). Radio Nature. Potters Bar: Radio Society of Great Britain. ISBN 9781-9050-8638-2.