During the joint heating campaign involving, UK (Lancaster), Germany
and Sweden on 16 February 1999, enhanced airglow at 6300 Å 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 Å
(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.
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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