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 (sqrt 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