Natural MASERS are now widely accepted as being responsible for the generation of various non-thermal electromagnetic emissions which are observed within, or emanating from magnetized planets or stars.
These phenomena are basically linked to electromagnetic instabilities due to resonant wave-particle interactions between the electromagnetic fields and energetic electronic populations characterized by strong non-equilibrium features. These features are themselves associated with energetization processes occurring at larger spatial scales: particle acceleration by localized potential structures or low frequency waves and particle precipitations.
The microphysics of these WPI's will be briefly reviewed with a particular emphasis on the analogy between the linear and non-linear evolution of these classical processes and those of the quantum mechanical MASERS.
A new insight into these problems has been allowed by recent numerical simulations, which aim as modelizing the complex phenomena with occur within the source regions of the terrestrial (Auroral) main radio emission.
Focussing on this last phenomenon, one will discuss the results of these computational works and compare them to both the above quoted theoretical analysis and to the observational situation. One will particularly study the saturation mechanism of these emissions, the true nature of which is, up to now, partly controversial.