Direct generation of the auroral kilometric radiation by the maser synchrotron instability: An analytical approach

Abstract

It has recently been shown that radio emission from the Earth, the so-called auroral kilometric radiation, is emitted inside cold plasma depleted regions. These observations have motivated a new interest in direct generation processes. Since it allows the generation of an X mode just above the cutoff frequency of this mode (in agreement with observations) the maser synchrotron instability is a promising candidate. A detailed analytical approach to the understanding of this instability is proposed. The complete dispersion relation is derived in a situation where the cold plasma density is small (εc=ω2pe/ω2c≪1). The physical significance of this instability is discussed; in particular, the difference between the maser synchrotron instability and the classical Weibel instability is stressed. Depending upon the ratio between ‘‘cold’’ and energetic electron components, two regimes, the ‘‘kinetic’’ and the monoenergetic ‘‘ring-like’’ distribution, are considered and the results are compared with the numerical solution of the dispersion relation. Finally the saturation process of this instability is discussed in a homogeneous infinite medium; it is shown that trapping effects can insure an efficient saturation of the maser synchrotron instability.