The Earth, Jupiter, Saturn, and Uranus are observed to emit intense radio waves, which include auroral kilometric radiation (AKR), Jovian decametric radiation (DAM), Jovian kilometric radiation (KOM), Satumian kilometric radiation (SKR), Uranian kilometric radiation (UKR), and terrestrial, Jovian, Saturnian, and Uranian continuum radiation. A number of theories have been proposed in the past years to explain the non-thermal radio emissions from these planets. A brief review of various radio emission mechanism is given in the paper.
The mechanisms of non-thermal emissions from planets can be divided into two classes: (A) direct emission mechanisms and (B) mode-conversion mechanisms. Each class can be subdivided into linear and nonlinear processes. In the direct emission mechanisms, the electromagnetic X-mode and O-mode waves are directly generated by plasma instabilities and escape from the source region. In the mode-conversion mechanisms, electrostatic waves are first produced and then converted into electromagnetic waves. In this review, we emphasize the cyclotron maser mechanism (a direct emission process) and the linear mode-conversion process.
The electron cyclotron maser process, which requires a population inversion in the electron velocity distribution, provides a very efficient means for the generation of radio emissions. The process appears to be the favored mechanism for generating auroral kilometric radiation and its analogues, Jovian decametric radiation, Satumian kilometric radiation, and Uranian kilometric radiation. In particular, the cyclotron maser process can explain the X-mode, O-mode, Z-mode, and second harmonic X-mode emissions observed above the auroral ionosphere. On the other hand, the linear mode-conversion of upper-hybrid waves to O-mode waves is the favored mechanism for the generation of the non-thermal continuum radiation observed in the magnetosphere of these planets.