Electromagnetic

Electromagnetism

Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

 hiss is a naturally occurring Extremely Low Frequency

Extremely low frequency

Extremely low frequency is a term used to describe radiation frequencies from 3 to 300 Hz. In atmosphere science, an alternative definition is usually given, from 3 Hz to 3 kHz...

/Very Low Frequency

Very low frequency

225px|thumb|right|A VLF receiving antenna at [[Palmer Station]], Antarctica, operated by Stanford UniversityVery low frequency or VLF refers to radio frequencies in the range of 3 kHz to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals...

 electromagnetic wave (i.e., 300 Hz – 10 kHz) that is generated in the plasma

Plasma (physics)

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 of either the Earth's ionosphere

Ionosphere

The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

 or magnetosphere

Magnetosphere

A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

. Its name is derived from its incoherent, structureless spectral properties which, when played through an audio system, sound like white noise

White noise

White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...

 (hence the onomatopoetic name, "hiss").

Varieties

Hiss may be observed in any of several varieties depending on local time and L-shell

L-shell

The L-shell, L-value, or McIlwain L-parameter is a parameter describing a particular set of planetary magnetic field lines. Colloquially, L-value often describes the set of magnetic field lines which cross the Earth's magnetic equator at a number of Earth-radii equal to the L-value...

 of the observer:

• Plasmaspheric hiss is generally observed within the plasmasphere
  Plasmasphere
  The plasmasphere, or inner magnetosphere, is a region of the Earth's magnetosphere consisting of low energy plasma. It is located above the ionosphere...
  
  , peaking in frequency slightly below ~1 kHz and rarely exceeding 3 kHz.
• Exo-hiss and ELF hiss are two varieties of hiss observed outside of the plasmasphere
  Plasmasphere
  The plasmasphere, or inner magnetosphere, is a region of the Earth's magnetosphere consisting of low energy plasma. It is located above the ionosphere...
  
  , both having a spectrum similar to that of plasmaspheric hiss.
• Midlatitude hiss is generally observed outside of the plasmasphere and tends to have frequencies between 2 and 10 kHz.
• Auroral hiss is observed in the auroral zones
  Aurora (astronomy)
  An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
  
   of the Earth and can extend up to several hundred kHz.

Generation mechanisms

There are several proposed generation mechanisms for plasmaspheric hiss in particular, including:

• Generation from discrete chorus
  Dawn chorus (electromagnetic)
  The electromagnetic dawn chorus is an unexplained phenomenon that occurs most often at sunrise or shortly after, that resembles the sound of the birds' dawn chorus. Chorus is believed to be generated by a Doppler-shifted cyclotron interaction between anisotropic distributions of energetic ...
  
   emissions
• Generation via electromagnetic impulses from terrestrial lightning
  Lightning
  Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
  
  , specifically via lightning-generated whistlers
  Whistler (radio)
  A whistler is a very low frequency electromagnetic wave which can be generated, for example, by lightning. Frequencies of terrestrial whistlers are 1 to 30 kHz, with maximum usually at 3 to 5 kHz. Although they are electromagnetic waves, they occur at audio frequencies, and can be...