1999ApJ...524..961S

Plasma radiation near the second harmonic of the plasma frequency driven by the loss-cone instability of magnetically trapped energetic electrons in stellar coronae is considered. Growth rates of longitudinal waves near the upper hybrid frequency are determined for warm background plasma and sufficiently high plasma densities, ω_p>ω_c, where the electrostatic instability prevails over the electromagnetic cyclotron maser instability, with particular attention given to the intermediate magnetic field condition, 1<ω²_p/ω²_c≲5. The plasma turbulence level and the brightness temperature of the second-harmonic plasma radiation arising from the coalescence of upper hybrid waves are estimated. The brightness temperature can reach ∼10¹⁴ K for spontaneous conversion of the waves and ∼10¹⁶ K for induced conversion. The radiation 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 window 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 intermediate magnetic field case. Because of the high temperature of the plasma in the 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 nonthermal quiescent and flare radio emission from active stars.