Parametric excitation of lower hybrid waves by Z-mode waves near electron cyclotron harmonics at Tromso

Z-mode wave converted from O-mode heater wave is considered as the pump wave in Tromso HF heating experiments for the parametric excitation of lower hybrid decay mode together with electron Bernstein sidebands in the region above the O-mode reflection layer. This is the process, suggested by Mishin et al. [1997], which generates superthermal electrons by the excited lower hybrid waves. These superthermal electrons produce plasma waves in the region above the O-mode reflection layer, with frequencies much greater than the heater wave frequency, as observed by Isham et al. [1990, 1996]. Our detailed analyses of this process are carried out for two experimental cases wherein the pump wave frequencies are near the third and fifth harmonic electron cyclotron frequencies, respectively. The results show that the minimum threshold field of the instability in either case is less than 0.2 V m^-1. However, because the wavelength of the instability is also short, the collisionless dampings restrict the instability occurrence to narrow wavelength and frequency ranges. For 1 V m^-1 pump field intensity, the instability can be excited in less than 1 ms. Since the excited lower hybrid waves are strongly Landau damped, superthermal electrons are efficiently generated by them.