Generation of the Auroral Kilometric Radiation in plasma cavities—II. The cyclotron maser instability in small size sources

One of the important results obtained with the Viking spacecraft was to show that the Auroral Kilometric Radiation (AKR) is generated in thin source regions where the plasma, hot and tenuous, is very different from the surrounding plasma, denser and colder. A model of the generation of the AKR by the cyclotron maser instability that takes into account the finite geometry of the sources is proposed. An equation of dispersion describing the inhomogeneity effects is established and solved for different parameter regimes (various densities inside and outside the source, various electron energies and source width). The properties of its solutions (quantification of the wave vector, characteristics teristics of the internal and external waves, etc.) are investigated. For the range of parameters adapted to the AKR generation, it is shown that the most unstable waves (on the internal X mode) cannot be directly connected to the external X mode waves. A more complex scenario than the direct connection between the internal unstable waves and the external X mode waves must then be invoked for explaining the escape of the electromagnetic energy on this external mode. A possibility could be that the waves are confined inside the source until they reach an altitude where they can be connected to the external X mode. To be efficient, this scenario implies that the transmission coefficients across the source frontiers (on the O and Z modes) are as low as possible. It is shown that this condition puts severe constraints on the radiating diagram, the only possibility being that the waves are generated in a direction tangent to the source frontiers. This constraint is in perfect accordance with the observations reported in the companion paper (Louarn and Le Quéau, Planet. Space Sci. 44, 199–210, 1996).