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During the joint heating campaign involving, UK (Lancaster), Germany and Sweden on 16 February 1999, enhanced airglow at 6300 was observed by HF pumping of the ionospheric F region plasma at auroral latitudes from the EISCAT Heating transmitter. The airglow was detected simultaneously by up to four imaging station of ALIS in northern Sweden (figure 3). Airglow enhancement occurs as a result of excitation of the O(1D) meta-stable state, which radiates at 6300 (the red line) as the oxygen atom relaxes to its ground state. Basically, two different mechanisms are proposed for the excitation of the electrons and subsequent airglow enhancement. In the first case it is assumed that the electron velocity distribution remains essentially Maxwellian and the energetic electrons are associated with the tail of the velocity distribution of the pump-enhanced electron temperature. In the second case the supra-thermal electrons belong to a non-Maxwellian distribution, which results from acceleration processes in Langmuir turbulence driven by the pump wave. EISCAT observation indicates large heater-induced electron temperature enhancements of up to 4500 K, or 450% of the unperturbed temperature which extends several tens of kilometers below and several hundreds of kilometers above the airglow cloud.

Airglow

Electron density and electron temperature versus altitude as measured by EISCAT-UHF radar, and the 630 nm emission intensity measured with ALIS. The measurements were made during the ionospheric heating experiment on 16 February 1999.

The short vertical extent of the airglow cloud and the high anomalous absorption of the pump wave that is expected for the large electron temperature enhancements suggest that the electron energisation which is needed to enhance the airglow occurs essentially perpendicular to the magnetic field and is due to upper hybrid turbulence.


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