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Observations of Fluxes of Suprathermal Electrons Accelerated by HF Excited Instabilities

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Journal of Atmospheric and Terrestrial Physics





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When sufficiently high power o-mode HF radio waves are transmitted at frequencies below the ionospheric maximum plasma frequency, not only are plasma instabilities excited, but ambient electrons are accelerated to energies of several electron volts (vs electron thermal energies of order 0.1 eV). Such fluxes have been experimentally inferred from enhancements of atomic oxygen 6300 Å, and occasionally 5577 Å airglow emission. Active theoretical work has led to conflicting theories for the basic electron acceleration mechanism responsible. We report here the most direct observations to date of such suprathermal electrons. During experiments at Arecibo, transmitting about 140 kW of o-mode power at 7.63 MHz, near but below the (exciting strong plasma wave instabilities near an altitude of 285 km) nighttime plasma line intensities were observed to be enhanced by a factor of 10–100. These enhancements extend over a much broader altitude range than that to which HF excited plasma instabilities are confined. When the transmitter was turned off, these plasma line intensities immediately relaxed to their thermal level. Data are presented which should help guide future theoretical development. One significant conclusion that the observations directly lead to is that electrons are accelerated up to at least 20 eV, an energy so high as to call for reassessment of existing published theories. The data also contradict a highly sensitive HF power dependence of the suprathermal electron flux. The production rate is relatively flat from 10 to 20 eV and relatively stable over several hours for constant HF power densities. Model calculations demonstrate that the observed altitude variation of the plasma line intensities is dominated by collisional energy degradation of the suprathermal electron flux to the neutral atmosphere and ionospheric plasma. The modeling also demonstrates that a valid treatment of the plasma physics involved must include some of the geophysical processes (e.g. scattering by the neutral atmosphere).


Published by Elsevier in Journal of Atmospheric and Terrestrial Physics. Publisher PDF is available through link above. Publisher requires subscription to access article.