Impact of vibrational excitation on ionospheric parameters and artificial airglow during HF heating in the F region

Abstract

Vibrational excitation by the impact of electrons on molecular nitrogen in the energy range of 1.8-3.2 eV is well known. Electrons heated by high-power HF radio waves can effectively lose energy due to this vibrational excitation, which, in tum, creates a sharp energy barrier. Electrons must overcome this barrier in order to reach higher energies. A model for this vibrational barrier has been developed, and deviations of the electron energy distribution from a Maxwellian distribution have been estimated under different conditions. The depletion of electrons with energy higher than 2 eV decreases the excitation rate of the optical emissions (for example, 630.0 nm airglow) observed during HF heating. We show that the vibrational barrier can explain why the 630 nm airglow reported by Gustavsson et al. [2001] was only slightly enhanced during HF heating, in spite of the measured electron temperature being higher than 3000 K. In some cases, vibrationally excited molecules can accumulate in the ionosphere due to slow deactivation. These species increase the recombination coefficient, which, in turn, may result in the decrease of electron density. This effect can also contribute to the saturation of the optical emission strength as a function of heater power, as reported by Pedersen et al. [2003]. Copyright 2004 by the American Geophysical Union.