Simulations of the Generation of Energetic Electrons and the Formation of Descending Artificial Plasma Layers During HF Heating at Arecibo

HF-induced Descending Artificial Plasma Layers (DAPLs) are artificially ionized plasma layers with plasma density in excess of that of the F2-peak. They were discovered during HF heating experiments at HAARP where they descended up to 70 km from the initial O mode wave reflection height. The DAPLs were attributed to the ionization of the neutral gas by high-energy electrons accelerated by the artificial ionospheric turbulence. Recently, DAPL formation was reported during the HF-heating experiment at Arecibo [Bernhardt et al., 2017]. This result was unexpected since Arecibo has the Effective Radiated Power (ERP) 4 – 5 times lower than that at HAARP, and since the experiment at Arecibo also has an unfavorable geometry, where the HF beam is directed vertically while the inclination of the geomagnetic field is 43.5o, allowing the fast electrons to escape the volume where their interaction with the artificial plasma turbulence occurs. However, the presence of photoelectrons due to the UV radiation from the Sun at the low latitude of Arecibo could magnify the flux of hot electrons. A model of artificial plasma layers created by the Arecibo HF facility is presented. It shows that Langmuir turbulence due to the HF heating can accelerate part of the ambient photoelectrons to energies above the ionization threshold of the neutral gas, leading to the formation of DAPLs. The present model results are in quantitative agreement with the experiments of Bernhardt et al. [2017].