On the linear stage of thermal parametric instabilities in the ionosphere excited by HF pumping near electron gyroharmonics

Thresholds and growth rates of the thermal parametric instability in the ionosphere were studied taking into account the regular inhomogeneity (altitude dependence) of the plasma density and of the magnetic field strength. A noticeable decrease of the pump wave threshold power and an increase of the growth rate occurs when f0 approaches the double resonance frequency fd = nfce(zd) = fUH(zd), zd being the altitude for double resonance. Particularly, the threshold power decreases relative to the non-resonant case by a factor 3–7 at |f0 − fd(zd)| ≲ 15 kHz for n = 3 and up to a factor 3 for n = 4. The instability growth rate can increase with a factor of 103–104 relative to the non-resonant case. The decrease of the threshold and increase of the growth rate occur for quite small scales of the striations, l⊥ ∼ 1 m for n = 3 and l⊥ ∼ 0.5 m for n = 4.