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Powerful HF electromagnetic waves transmitted into the ionosphere from the High-frequency Active Auroral Research Program (HAARP) facility in Alaska and the European Incoherent Scatter Scientific Association (EISCAT) facility in Norway have induced artificial aurora with ground-measurable 4278 A wavelength emissions (B. Gustavsson et al., Ann. Geophys., 23, 1747–1754, 2005; T. B. Pedersen et al., Geophys. Res. Lett. 37, 2, L02106, 2010). This artificial “blue-line” emission can result from the electronic transition of an N 2 +(1N) ion to its ground state. The two main sources of N 2 +(1N) ions in the F region are photoionization and electron-impact ionization of N 2 molecules. Experimental and theoretical evidence suggests that impact ionization of N 2 molecules by electrons accelerated by wave-plasma interactions to energies exceeding 18 eV is responsible at least in part for artificial 4278 A wavelength emissions observed during ionosphere radio modification experiments at HAARP and EISCAT (H. C. Carlson et al., J. Atmos. Solar Terr. Phys., 44, 12, 1089–1100, 1982; A. V. Gurevich et al., J. Atmos. Terr. Phys., 47, 11, 1057–1070, 1985). |
