Naturally enhanced radar spectra and low energy auroral emission

Sullivan, J.1, Lockwood, M.2,3, Lanchester, B.3, Kontar, E.4 and Whiter, D.3

1 Space Environment Physics Group, University of Southampton, UK
2 Rutherford Appleton Laboratory, Didcot, UK
3 University of Southampton, UK
4 Department of Physics and Astronomy, University of Glasgow, Glasgow, UK

We present a study of EISCAT Svalbard radar data in which many examples of anomolous spectral events known as naturally enhanced ion-acoustic lines, or NEIALs, were observed. NEIAL activity occurred in the polar cusp region under lobe-reconnection conditions during an extended period of low solar wind density within an extremely high speed solar wind stream.

The Andor imager, fitted with a narrow passband filter around 7325 Å and primarily sensitive to the forbidden [OII] doublet emission, and the SIF imager, responding mainly to N2 1PG emissions, were both set up as part of the Spectrographic Imaging Facility in Adventdalen during this period. Using these two imagers we show a direct link between the occurrence of multiple NEIAL events and co-located auroral emissions caused specifically by extremely low-energy electron precipitation i.e. less than about 100eV. This observation is consistent with the parametric instability model for NEIAL generation, where beam excited langmuir waves decay down to ion-acoustic wave modes through a 3 wave interaction.

In selected events studied, where a clear brightening in the aurora is associated with the occurrence of a NEIAL, processed images show field aligned auroral filaments, seen in the sub-100eV imager, passing through the centre of the radar beam. Possible links with low energy electrons observed at the open-closed boundary by a DMSP satellite passing near the ESR are discussed.