Sammendrag
Anomalous incoherent scatter echoes are radar echoes from the F or upper E regions whose ion line spectral shape departs from that corresponding to thermal equilibrium. The scattered power is enhanced and the spectra can be symmetric or asymmetric with one or the other shoulder more enhanced. Several theories have been proposed which can be divided into two classes: (1) direct electron or ion beam instabilities that distort and enhance the ion line and (2) a beam-enhanced Langmuir wave which becomes the pump for a parametric decay into enhanced daughter Langmuir and ion acoustic waves; the beam-enhanced pump and the enhanced daughter waves may be observed as non-thermally enhanced incoherent scatter ion and plasma lines. Direct beam instabilities fail to explain one or another characteristic of the observations, for example they may require large currents or enhance only one ion line shoulder, contrary to observations showing enhancements of either or both shoulders, or both with a central peak. On the other hand, parametric instability mechanisms appear better positioned to explain most if not all of the observations. Since they also produce enhancements of the plasma lines, a decisive test is whether the anomalous ion line echoes are accompanied by enhanced plasma lines. Parametric instabilities, or Langmuir turbulence, can also explain the enhanced central ion line peak. Here we present recent measurements made with the EISCAT VHF (224 MHz) radar in northern Scandinavia of anomalous ion line spectra accompanied by strong plasma line enhancements. These measurements provide important evidence in support of the parametric decay instability, or, more generally, of natural Langmuir turbulence in the aurora.
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