Second-harmonic plasma radiation of magnetically trapped electrons in stellar coronae

Plasma radiation near the second harmonic of the plasma frequency driven by the loss-cone instability of magnetically trapped energetic electrons in s tellar coronae is considered. Growth rates of longitudinal waves near the u pper hybrid frequency are determined for warm background plasma and suffici ently high plasma densities, omega(p) > omega(c), where the electrostatic i nstability prevails over the electromagnetic cyclotron maser instability, w ith particular attention given to the intermediate magnetic held condition, 1 < omega(p)(2)/omega(c)(2) less than or similar to 5. The plasma turbulen ce level and the brightness temperature of the second-harmonic plasma radia tion arising from the coalescence of upper hybrid waves are estimated. The brightness temperature can reach similar to 10(14) K for spontaneous conver sion of the waves and similar to 10(16) K for induced conversion. The radia tion pattern of the second-harmonic plasma emission is also calculated; it shows a prevalence of the extraordinary mode. Analyzing the problem of the escape of radiation from stellar coronae, it is found that the escape windo w is wider for the o-mode because the x-mode radiation is strongly absorbed by the warm background plasma at the low harmonic gyrolevels, and thus the observed radiation can be polarized in the ordinary sense in the intermedi ate magnetic field case. Because of the high temperature of the plasma in t he coronae of X-ray-emitting stars, the characteristic length scale of the wave conversion and the efficiency of the plasma radiation mechanism can be much higher than on the Sun. The results are discussed in the context of n onthermal quiescent and flare radio emission from active stars.