Electromagnetic radiation from upper-hybrid wave turbulence in inhomogeneous solar plasmas

Type III solar radio bursts have been commonly observed in the solar wind and coronal plasmas. Electron beams accelerated in the Sun's atmosphere generate weakly magnetized Langmuir (upper-hybrid) wave turbulence producing in turn electromagnetic emissions at the electron plasma frequency ω _p and its harmonic 2ω _p, via successive processes involving interactions between the background plasma, the waves and the beam particles. The impact of the random density fluctuations inherent to the background plasma on these processes requires the development of novel approaches and models. Owing to a new theoretical modeling, the radiation efficiency of electromagnetic waves radiated at ω _p from a plasma source with random density fluctuations and developed upper-hybrid wave turbulence is calculated analytically and numerically. It is shown that the maximum radiation efficiency at frequency ω _p scales as the average level of density fluctuations and as the ratio (c/v_T)^–2 (where v_T is the thermal velocity). These scaling laws are found owing to two different and novel methods of determination of the electromagnetic radiation by turbulent inhomogeneous plasma sources through waves' transformations on randomly fluctuating density irregularities. These results contribute significantly to the understanding of the processes involved in the generation of Type III solar radio bursts. Their presentation is preceded by a summary of previous studies on the dynamics of beam-driven Langmuir turbulence in inhomogeneous solar plasmas. (paper)