EXCITATION OF OSCILLATING 2 STREAM INSTABILITY BY UPPER HYBRID PUMP WAVES IN IONOSPHERIC HEATING EXPERIMENTS AT TROMSO

It is shown that upper hybrid waves generated by the HF heater wave in the Tromso' heating experiments can excite oscillating two stream ins tability near the upper hybrid resonance layer of the heater wave. The sidebands of the investigated parametric instability are obliquely pr opagating Langmuir waves. These Langmuir waves can be excited in milli seconds and have a broad angular distribution. Thus, those waves propa gating obliquely at small angles with respect to the geomagnetic field can contribute to the zero off-set frequency component of the plasma line spectrum detected by a backscatter radar. The analyses also show that the wavelengths of the Langmuir waves have an upper bound and con sequently, explain why the zero off-set frequency plasma line has been detected by the EISCAT 933 MHz (UHF) radar, but riot by the EISCAT 24 4 MHz (VHF) radar as reported in Stubbe et. al. [1992].