Abstract
Abstract. Optical data are compared with EISCAT radar observations of multiple Naturally Enhanced Ion-Acoustic Line (NEIAL) events in the dayside cusp. This study uses narrow field of view cameras to observe small-scale, short-lived auroral features. Using multiple-wavelength optical observations, a direct link between NEIAL occurrences and low energy (about 100 eV) optical emissions is shown. This is consistent with the Langmuir wave decay interpretation of NEIALs being driven by streams of low-energy electrons. Modelling work connected with this study shows that, for the measured ionospheric conditions and precipitation characteristics, growth of unstable Langmuir (electron plasma) waves can occur, which decay into ion-acoustic wave modes. The link with low energy optical emissions shown here, will enable future studies of the shape, extent, lifetime, grouping and motions of NEIALs.
Highlights
Enhanced Ion-Acoustic Lines (NEIALs) have been seen in Incoherent Scatter Radar (ISR) data for almost 20 years, as an increase in received power of up to 3 orders of magnitude above the normal thermal level
A clear link has been shown between Naturally Enhanced Ion-Acoustic Line (NEIAL) in the dayside cusp and emission from low-energy (100 eV or less) electrons, using auroral imaging in the forbidden (OII) emission doublet
The results from cameras at two different wavelengths imply that this emission is due to low energy primary electron precipitation, and not secondaries linked to incoming particles of higher energy
Summary
Enhanced Ion-Acoustic Lines (NEIALs) have been seen in Incoherent Scatter Radar (ISR) data for almost 20 years, as an increase in received power of up to 3 orders of magnitude above the normal thermal level. From a number of proposed theories, “parametric decay” has emerged in recent studies as the most likely candidate for the mechanism responsible for NEIAL observations in the dayside cusp In this interpretation, low energy electron beams cause the growth of unstable Langmuir (electronplasma) waves through the bump-on-tail instability; these waves decay to ion-acoustic wave modes at ISR wavelengths (Forme, 1993, 1999; Kontar and Pecseli, 2005). There are indications that night-side NEIALs are often seen near the edges of structured auroral arcs, as opposed to within filamentary rays (Collis et al, 1991; Michell, 2007) This suggests the possibility that dayside and nightside echoes may be the result of differing generation processes, but with similar signatures in the radars. The DMSP data greatly enhances our ability to investigate directly driving conditions at NEIAL altitudes, as well as using signatures from optical measurements up to 300 km in altitude
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