Microwave transition radiation in solar flares and in astrophysics

We consider the role of transition radiation in astrophysical sources of radio emission. The principal results of the theory of transition radiation by relativistic particles in a magnetoactive turbulent plasma are presented. Such plasmas are frequently encountered in astrophysical conditions, particularly in solar flares. We show that transition radiation in solar flares is generated most effectively in the frequency range f ∼ (0.5-10) GHz with fluxes possibly exceeding 1000 solar flux units. We show that transition radiation can be the main radiation mechanism in both weak and strong solar flares at certain frequency bands in dense (ω_Be ω_p) plasmas. In strong magnetic fields (ω_Be ≫ ω_p) it is generated only by protons because that of the electrons is strongly suppressed. In this case the polarization of the transition radiation corresponds to the O-mode. However, we find that the emission from relativistic protons in flares as a rule is not significant. We discuss the possibility of using observations of transition radiation for diagnostics of small-scale inhomogeneities of the electron background plasma density, 〈Δn_e²〉/n_e².