Sammendrag
An event was measured by the VHF EISCAT radar containing enhanced
upshifted and downshifted Langmuir and ion acoustic waves plus a zero
frequency enhancement all occurring simultaneously and in the same
altitude interval. The event lasted about 50 seconds and had an altitude
span of about 14 km. It was coincident with auroral precipitation as
evidenced by electron density measurements made by the radar and by
optical observations. The theory of François Forme
(Ann. Geophysicae, Vol 17, p. 1172, 1999)
can explain these measurements whereby a beam of electrons excites
Langmuir waves via the bump in tail instability which subsequently decay
into other Langmuir waves and ion acoustic waves. The excitation is
sufficiently strong to reach the regime of the modulational instability
hence producing an enhancement at zero frequency as well. It is of
interest to emphasize that this mechanism is the same as the one that
accounts for the wave enhancements observed during heating experiments as
well as the waves that produce solar radio bursts of type III. The
Zakharov equations offer a model framework to explain the three phenomena.
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