We discuss results on plasma density profile modifications in the F-region ionosphere caused by HF heating with the frequency f0 in the range [(-150 kHz)-(+75 kHz)] around the 4th electron gyroharmonic 4fc. The experiments were conducted at the HAARP facility on June 2014. The multi-frequency Doppler Sounder (MDS) which measures the phase and amplitude of reflected sounding radio waves complemented by the observations of the Stimulated Electromagnetic Emission (SEE) were used for the diagnostics of the plasma perturbations. We detected noticeable plasma expulsion from the reflection region of the pumping wave, and from the upper hybrid region, the expulsion from the latter one had been strongly suppressed for f0≈4fc. The plasma expulsion from the upper hybrid region was accompanied with the sounding wave’s anomalous absorption slower developing for f0≈4fc. Also, slower developing and weaker expulsion was detected for the height region between the pump wave reflection and upper hybrid altitudes.

The Arecibo Observatory (AO) could modify the ionosphere using high frequency (HF) waves. During the HF experiments, the incoherent scatter radar (ISR) was used to study the behavior of the ion, plasma, and gyro lines with 150m height resolution. One year ago, the AO platform collapsed and put a pause for new experiments. However, the archived ISR data can answer open questions like the electron heating evolution in the interaction region. This paper presents a new methodology to estimate the electron temperature (Te) at the resonance altitude based on the physics of the HF wave-plasma interaction. Estimating Te inside regions where the ionospheric plasma interacts with the powerful HF ground waves is a challenge. Standard ISR techniques to assess the temperatures using the ion line are based on Maxwellian approximations. However, the irregularities generated by HF experiments induce non-Maxwellian behaviors. Therefore, a new approach is proposed using the ion-acoustic phase velocity (C_ia) of the ion-acoustic waves generated during the HF experiments. The ion acoustic velocity can be derived from the ISR enhanced plasma line (HFPL) produced during active experiments. The HFPL is mainly attributed to the HF wave decaying into a cascade of Langmuir and ion-acoustic waves, known as Parametric Decay Instability (PDI). The ion-acoustic waves travel at speed: C_ia=λf_ia, where f_ia is the ion-acoustic frequency, and λ is the radar (Bragg backscatter) wavelength. The PDI signature is characterized at the HFPL by cascaded lines spaced in frequency by multiples of f_ia. After measuring f_ia, Te is obtained using f_ia=1/λ √(k_B (Te+2Ti)/m_i), where Ti and mi are the ion temperature and mass. Estimates for one particular experiment on June 12, 2019 show that Te is usually higher at the top of the layer and the beginning of every HF pulse. For example, at 280s after 16:43:00LT, it reached a value near 3500 K, when the temperature outside of the interaction region was below 1600 K.

The first demonstration of rocket exhaust driven amplification (REDA) of whistler mode waves occurred on 26 May 2020 by transferring energy from pickup ions in a rocket exhaust plume to EM waves. The source of coherent VLF waves was the Navy NML Transmitter at 25.2 kHz located in LaMoure, North Dakota. The topside ionosphere at 480 km altitude became an amplifying medium with a 60 second firing of the Cygnus BT-4 engine. The rocket engine injected exhaust as a neutral cloud moving perpendicular to field lines that connected the NML transmitter to the VLF Radio Receiver Instrument (RRI) on e-POP/SWARM-E. Charge exchange between the ambient O+ ions and the hypersonic water molecules in the exhaust produced H2O+ ions in a ring-beam velocity distribution. The 25.2 kHz VLF signal from NML was amplified by 30 dB for a period 77 seconds as observed by the RRI. Simultaneously, preexisting coherent ELF waves at 300 Hz were amplified by 50 dB during and after the Cygnus burn. Extremely strong coherent emissions and quasi-periodic bursts in the 300 to 310 Hz frequency range lasted for 200 seconds after the release. The excitation of an ELF whistler cavity may have lasted even longer, but the orbit of the SWARM-E/e-POP moved the RRI sensor away from the wave emission region. The amplified 300 Hz ELF waves may have gained even more energy by cyclotron resonance with radiation belt electrons as they were ducted between geomagnetic-conjugate hemispheres.