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Using electron density and temperature equations, we have modeled the dynamics of the electron density profile in the ionosphere due to the expulsion of plasma from localization regions of plasma waves, pumped by high-power HF radio waves, i.e. wave reflection and upper hybrid resonance regions. Causes of the ionospheric plasma expulsion are an increase in the gas-kinetic pressure due to the ohm heating of electrons by plasma waves, and the high-frequency pressure of plasma waves (ponderomotive expulsion). We have established that the ponderomotive expulsion develops more rapidly and is responsible for the formation of local regions of plasma density depletion near plasma resonances, whereas the gas-kinetic pressure increase is responsible for the formation of lower-density region, which is slower in time and more extended and smoother in height. The results obtained qualitatively agree with the data from the experiment conducted at the HAARP facility in 2014.

Many phenomena in the high‐frequency pumped ionosphere exhibit dependence on the pump beam incident angle α. This motivates a systematic study of the α dependence of the stimulated electromagnetic emission (SEE), particularly near electron gyroharmonics. We report the first observations of stationary SEE spectra for α ranging from −28° (north) to 28° (south) at three receiving sites, for the pump frequency (f0) sweeping near the fourth gyroharmonic (4fc). The following is established: (i) For pumping near the magnetic zenith (α = 7°, 14°, 21°), when existent dynamic broad upshifted maximum in the SEE spectrum indicates that artificial ionization layers are excited, suppression of the downshifted maximum at f0 ≈ 4fс is weakest, and 4fс increases. (ii) Weaker similar effects occur for α = −14° (a condition called "mirror magnetic zenith"). (iii) For northern pump beam inclinations, the SEE intensity decreases (in comparison with southern inclinations and vertical), most strongly at the southernmost receiving site. Plain Language Summary: Many phenomena in the high‐frequency (HF)‐pumped upper ionosphere, such as artificial airglow, descending artificial ionization layers, pump‐induced plasma irregularities, and HF plasma modes, exhibit dependence on the pump beam incident angle with respect to the ionosphere. This motivates the systematic study of angle dependence of the stimulated electromagnetic emission (secondary radiation from the ionosphere) appearing due to plasma mode conversion into electromagnetic ones, particularly near electron gyroharmonics. We report the first observations of secondary radiation spectra as dependent on mismatch between the pump frequency and fourth gyroharmonic, for incident angle varying from −28° (north) to 28° (south) at different receiving sites. The results obtained indicate that the interaction between high‐power high‐frequency pump wave and plasma modes in the ionospheric plasma takes place in a wide range of pump beam inclination angles; the interaction is stronger for southern inclinations. Newly discovered emission peculiarities, such as the emission suppression decrease at the fourth gyroresonance and energy redistribution between different emission spectral components (both for pumping to the magnetic zenith, 14°, when artificial ionization occurs) and similar but weaker effects for "mirror magnetic zenith" (−14°), allow deeper understanding of the physical processes occurring in the HF‐pumped upper ionosphere. Key Points: Properties of the stimulated electromagnetic emission are studied for different pump wave beam inclinations near the fourth gyroharmonicFor pumping at magnetic zenith, a correlation between emission spectral lines and artificial ionization layers is obtainedFor northern pump beam inclinations, the emission intensity decreases, most strongly at the southernmost receiving site [ABSTRACT FROM AUTHOR]

This paper describes the dynamic characteristics of ionospheric plasma turbulences excited by high‐power high‐frequency radio waves during ionospheric modification experiments at the High‐frequency Active Auroral Research Program heating facility, Gakona, Alaska. The data come from measurements of high‐frequency‐induced stimulated electromagnetic emission (SEE) and anomalous absorption of test radiowaves. In addition to quasi‐continuous pump (QCP) waves that excite the "standard" SEE, short pulses of diagnostic waves at frequencies shifted from the QCP frequency have been injected to generate diagnostic SEE. This experimental setup allowed us to study the spectra and damping rates of the plasma waves related to both diagnostic and QCP waves, as well as the QCP‐related small‐scale magnetic field‐aligned irregularities at different altitudes, ±25 km around the center of the heated volume. The results of numerical simulations of the upper hybrid‐related SEE and diagnostic SEE under a "double‐transformation" scheme with the use of an empirical model of the irregularities' spatial spectrum and its dynamics are in good agreement with the observations. Key Points: Artificial ionospheric turbulence dynamics is studied at the High‐frequency Active Auroral Research Program facilityAltitude size of the region occupied by pump‐induced small‐scale striations and their scales across the geomagnetic field are estimatedDamping rates of pump‐induced Langmuir and upper hybrid waves are determined [ABSTRACT FROM AUTHOR]