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Narrowband stimulated electromagnetic emissions (NSEE), a component of radio emissions created during high‐frequency (HF) radiowave ionospheric modification experiments, occur within 1 kHz of the HF pump frequency. NSEE was observed and studied for the first time at the HAARP (High Frequency Active Auroral Research Program) facility in Alaska (Norin et al., 2009,https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010,https://doi.org/10.1103/PhysRevLett.104.165004). Magnetized stimulated Brillouin scatter (MSBS) is a component of NSEE, which was also first observed at HAARP (Norin et al., 2009,https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010,https://doi.org/10.1103/PhysRevLett.104.165004) and later at European Incoherent Scatter Scientific Association (Fu et al., 2015,https://doi.org/10.5194/angeo-33-983-2015). Ion‐acoustic and electrostatic ion cyclotron modes are the daughter products of MSBS and can be used for the determination of the electron temperature and of the presence of minor ion species, respectively, in the HF‐modified ionosphere. Here we present the first observations of the MSBS process at magnetic midlatitudes, excited during radio wave ionospheric modification experiments at the Arecibo Observatory. The NSEE observations, in combination with a theoretical model and the wave matching conditions, are used to estimate background ionospheric parameters. A qualitative comparison of the MSBS component of the NSEE spectrum with the thermal ion line measured by incoherent scatter radar is presented.

Due to the potential for new diagnostic capabilities, there has been renewed interest in the generation of ionospheric stimulated electromagnetic emissions (SEEs) near the second harmonic of the pump frequency (ω₀), a process known as second harmonic generation (SHG). Observations of SHG during experiments at the High Frequency Active Auroral Research Program facility in whichω₀was stepped near the third harmonic of the electron gyrofrequency, 3ω_ce, and the transmit power linearly increased over the heating cycle at eachω₀, were reported recently. A key observation was the linkage between SEEs within ±30 Hz ofω₀, due to stimulated Brillouin scatter, and within ±30 Hz of 2ω₀. This current work reports further High Frequency Active Auroral Research Program observations that compare the time evolution of SEEs including SHG under the following two transmit power conditions (I) linear power ramp (II) maximum available power (2.8 MW). During these experiments,ω₀was stepped near 3ω_ceand also 2ω_ce. The results show that SEEs within ±100 Hz ofω₀and 2ω₀are both suppressed within a few seconds when the ionosphere is irradiated with the maximum available power. These SEEs appear to be suppressed before the onset of field‐aligned irregularities at the upper hybrid layer which is not in line with previous reports. Thus, other mechanisms, which are discussed, could possibly be responsible for the observed suppression of stimulated Brillouin scatter and SHG. Some preliminary diagnostics are derived from the SHG spectra temporal evolution by leveraging concepts from the field of laser plasma interactions.

Many phenomena in the high‐frequency pumped ionosphere exhibit dependence on the pump beam incident angleα. This motivates a systematic study of theαdependence of the stimulated electromagnetic emission (SEE), particularly near electron gyroharmonics. We report the first observations of stationary SEE spectra forαranging from −28° (north) to 28° (south) at three receiving sites, for the pump frequency (f₀) sweeping near the fourth gyroharmonic (4f_c). 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 atf₀≈ 4f_сis weakest, and 4f_сincreases. (ii) Weaker similar effects occur forα= −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.