Experimental Determination of Threshold Powers for the Onset of HF-Enhanced Plasma Lines and Artificial Ionization in the Lower F-Region Ionosphere

HF heating experiment, with linear power ramp from 50 to 92 dBW, was conducted to determine the HF threshold power levels for the onset of detectable plasma waves and the onset of artificial ionization inferred from HF-enhanced plasma lines (HFPLs). The threshold free-space electric field at 200-km altitude, where HFPLs were detected, is about 53 mV/m. However, the HF electric field near the reflection height is enhanced by a swelling factor ~ 3.76 and also conversion to a linear dipole pump mode (√2 ); the actual threshold field is about 281 mV/m that is consistent with theory. Artificial ionization in the lower region was detectable via UHF radar. Short wavelength upper-hybrid waves, which were excited parametrically by the HF heating wave at higher threshold, implement Doppler shifted harmonic-cyclotron resonance interaction, via finite Larmour radius effect, to effectively accelerate electrons. Monitoring the spectral power of the parametric decay instability (PDI) line in the HFPLs by radar was shown to determine the artificial ionization onset time more precisely than that by observing the start of a sharp downward trend in the altitude of the HFPLs; at this time, the HF free-space electric field slightly below 200-km altitude is about 550 mV/m. Langmuir cascade lines in the HFPLs are separated by intervals about double the ion-acoustic frequency (about 4-5 kHz) generated by the PDI. These lines, observed at lower power facilities, were not observed. The upper-hybrid OTSI and PDI excited at HighPower Active Auroral Research Program (HAARP) and the mode competition nonlinear-damping mechanism are suggested as the processes, suppressing cascade enhanced HFPLs.