Nonthermal Radio Emission

The classic sources of nonthermal radio emission within the solar system are Jupiter s magnetosphere and the solar corona. Nonthermal radio emissions have also been observed from the Earth s magnetosphere and the other three giant planets, Saturn, Uranus, and Neptune. The emissions from the latter planets can only be observed from spacecraft near the planets. Cyclotron and coherent plasma emission account for much of the low-frequency ( 10 MHz) nonthermal emission from these planets and the solar corona. This emission is highly variable and the details of the generation processes involved are not clearly understood. 

Synchrotron radiation is the dominant source of emission from Jupiter from about 50 MHz to 5 GHz it also accounts for continuum bursts of type IV from the sun. The theory of synchrotron radiation is well developed. A review article on magnetospheric radio emissions by Carr, Desch, and Alexander, contained in the book edited by Dessler (1983), lists a number of references. 

A single charged electron moving in a magnetic field is accelerated unless its velocity is solely in the direction of the magnetic field. This causes the electron to emit electromagnetic waves. The nature of these waves depends on whether the electron is nonrelativistic (velocity 3 x 10 ° cm/sec) or relativistic (velocity 3 x 10 ° cm/sec). The radiation emitted by nonrelativistic electrons is referred to as cyclotron radiation radiation emitted by relativistic electrons is referred to as synchrotron emission. 

Cyclotron radiation is emitted in all directions and has a frequency equal to the gyrofrequency. The radiation is polarized with the polarization depending on the direction of propagation. The polarization is circular when viewed along the direction of the magnetic field and linear when viewed in the plane of the orbit. At intermediate angles the polarization is elliptical. 

Relativistic electrons radiate not only at the gyrofrequency, but also at the harmonics. The relativistic mass increase with energy causes the harmonic spacing to decrease with increasing energy until the synchrotron spectrum is essentially smeared into a continuum. The radiation from a relativistic electron is highly nonisotropic. The emitted radiation is concentrated within a narrow cone about the instantaneous direction of the velocity vector with an approximate half-cone-width given by 

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Nonthermal radio emission Radio emission produced by processes other than thermal emission. Cyclotron and synchrotron radio emission are two examples of nonthermal radio emission. 

Because of the faintness of its radio emission, Pluto was the last planet to be detected. A few asteroids, satellites, and comets have been measured as well. Nonthermal radio emission has been measured from Jupiter, Saturn, Uranus, Neptune, and Earth. In this article we give an overview of the techniques used by planetary radio astronomers and discuss what has been learned from the measurements and what can be done in the future. In the interest of brevity, we do not discuss specific observations of asteroids and satellites, although they rightfully belong in any discussion of planetary radio astronomy. 

FIGURE 14 Three-dimensional tomographic reconstruction of Jupiter s nonthermal radio emissivity. The planet itself Is shown as a black sphere in this visualization. [After de Pater, I., and Sault. (1998). J. Geophys. Res. P/anefs 103(E9), 19,973-19,984.]... 

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