GENERATION OF THE AURORAL KILOMETRIC RADIATION IN PLASMA CAVITIES .2.THE CYCLOTRON MASER INSTABILITY IN SMALL-SIZE SOURCES

One of the important results-obtained with the Viking spacecraft was t o show that the Auroral Kilometric Radiation (AKR) is, generated in th in source regions where the plasma, hot and tenuous, is very different from the surrounding plasma, denser and colder. A model of the genera tion of the AKR by the cyclotron maser instability that takes into acc ount the finite geometry of the sources is proposed. An equation of di spersion describing the inhomogeneity effects if established and solve d for different parameter regimes (various densities inside and outsid e the source, various electron energies and source width). The propert ies of its solutions (quantification of the wave vector, characteristi cs of the internal and external waves. etc.) are investigated. For the range of parameters adapted to the AKR generation, it is shown that t he most unstable waves (on the internal X mode) cannot be directly con nected 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 el ectromagnetic energy on this external mode. A possibility could be tha t the waves are confined inside the source until they reach an altitud e where they can be connected to the external X mode. to be efficient, this scenario implies that the transmission coefficients across the s ource frontiers (on the O and Z modes) are as low as possible. It is s hown that this condition puts severe constraints on the radiating diag ram, the only possibility being that the waves are generated in the di rection tangent to the source frontiers. This constraint is in perfect accordance with the observations reported in the companion paper (Lou arn and LeQueau, Planet. Space Sci. 44, 199-210, 1996).