Discrete Electromagnetic Emissions in Planetary Magnetospheres

Roger R Anderson; William S Kurth

doi:10.1029/GM053p0081

A common feature of electromagnetic plasma instabilities in planetary magnetospheres explored to date is the occurrence of discrete emissions. The first concentrated studies of whistlers and discrete emissions were conducted using ground based very‐low‐frequency receivers in the early part of this century. Many of these emissions were given names such as “chorus”, “risers”, and “hooks”, which attempted to describe the sound of the received signals when they were routed through an amplifier and speaker system. We now know that the majority of these emissions are generated by whistler‐mode plasma instabilities in the Earth's magnetosphere. Despite the advanced state of our understanding of plasma instabilities, the reasons that the waves organize themselves into intense narrowband wave packets and the detailed explanation of the frequency variations are still the subject of study (experimentally, theoretically and by computer simulation) and debate. Discrete whistler‐mode “chorus” emissions have been observed in the magnetospheres of Earth, Jupiter, Saturn, and Uranus. Although differing in some details, the basic character of the emissions, consisting of a series of narrow‐band tones each rising or falling in frequency on a time scale ranging from tenths of a second to several seconds, is essentially the same. Another type of discrete emission called “lion roars” also occurs in the Earth's magnetosheath with characteristics very similar to chorus except it tends to occur at a lower frequency. Typical energies for electrons resonating with waves from these various discrete emissions range from a few hundred eV to as high as several hundred keV. In the Earth's magnetosphere discrete whistler‐mode emissions have been shown to play an important role in the pitch‐angle scattering and loss of energetic electrons from the radiation belt. Electromagnetic ion cyclotron waves in the Earth's magnetosphere have intense narrow‐band features similar in some respects to chorus. At frequencies above both the electron cyclotron frequency and the electron plasma frequency, terrestrial auroral kilometric radiation, which propagates predominantly in the free space (R‐X) mode, has a very complex frequency‐time structure consisting of many discrete emissions. A similar type of radio emission at Jupiter called S‐burst has been studied for many years by radio astronomers and is characterized by discrete tones drifting downward in frequency. Whether these free space mode emissions are fundamentally similar to the discrete ion cyclotron and whistler‐mode emissions is an open question. Discrete electromagnetic emissions have also been observed to be triggered in the magnetosphere by signals from ground transmitters as well as from lightning generated whistlers. Numerous active experiments, including ground transmitters, ionospheric heaters, and artificially injected particle beams and plasma, have produced discrete electromagnetic emissions in the Earth's ionosphere and magnetosphere. Both the observations and interpretations of discrete emissions will be reviewed.

Discrete Electromagnetic Emissions in Planetary Magnetospheres

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