Recovery of the ponderomotive parametric instability after long pumping of the ionosphere

Abstract We show that during high frequency (HF) pumping of the overhead ionosphere with a pump wave frequency slightly above the local fifth electron gyroharmonic, when one switches the pump wave from continuous pumping to low duty cycle pulse pumping, the stimulated electromagnetic emission (SEE) features related to the ponderomotive parametric instability (PPI) near the pump wave reflection point, such as the ponderomotive narrow continuum and the narrow continuum maximum, recover and become dominating in the SEE spectra after a few seconds after the switch. Simultaneously, the ionospherically reflected pump wave signal exhibits a recovery of ponderomotive self-action and the damped quasi-periodical oscillations (spikes) of the reflected pump wave, also related to the PPI development. The intensity of the down-shifted maximum (DM) feature in the SEE spectra increases for about 1–2 s after the switch to the pulse mode, but later, together with PPI related phenomena recovery, the DM intensity decreases noticeably. The phenomena described are attributed to the relaxation of small-scale irregularities (striations). The latter provide strong scattering of the pump wave into upper hybrid waves, strong shielding of the pump wave reflection point, and generation of the upper-hybrid related DM feature.