Abstract
Abstract. In this paper we present Naturally Enhanced Ion Acoustic Lines (NEIALs) observed with the EISCAT Svalbard Radar (ESR) together with auroral emissions observed with the Meridian Scanning Photometer (MSP). This is the first report of NEIALs observed during nighttime at Svalbard. Previously, NEIALs have been associated with a strong red line intensity (>10 kR), which exceeds the green line intensities. The high intensity in the red line emission is a sign of abundant low energy electron precipitation. In our observations, one of the NEIAL events was accompanied by the red line emissions far below the previously reported intensities. This happened when the green line intensity exceeds the red line intensity. In this work we discuss the behaviour of electron precipitation characteristics and optical emissions during NEIAL events on the nightside, and we suggest that intensity enhancements in the 844.6 nm emission line could be a better candidate than the 630.0 nm emission as an optical signature for NEIALs.
Highlights
Enhanced Ion Acoustic Lines are distinguishable from the standard thermal incoherent scatter (IS) spectrum due to the very strong enhancement, up to 4–5 orders of magnitude above the thermal level (Grydeland et al, 2004), of one or both ion acoustic shoulders in the power spectrum of the returned signal, see Fig. 1
This paper shows that Naturally Enhanced Ion Acoustic Lines (NEIALs) are a phenomenon which may be observed during nighttime hours at high latitudes in Svalbard
The geophysical settings indicate that the NEIAL events in this case study are associated with the substorm expansion phase on the nightside
Summary
The first observation of NEIALs in the dayside cusp/cleft region using the EISCAT Svalbard Radar (ESR) was reported by Buchert et al (1999). Sedgemore-Schulthess et al, 1999; Blixt et al, 2005) and enhanced plasma lines (Strømme et al, 2005) In all these studies, the observations of NEIALs were interpreted as being associated with particle precipitation, directly or indirectly. Rietveld et al (1991) observed NEIALs during severe geomagnetic disturbances with auroral particle precipitation, indicated by electron density enhancement in the E-region. Sedgemore-Schulthess et al (1999) found that NEIAL spectra occur during observations of poleward moving auroral transients and auroral coronal forms, and Blixt et al (2005) found a strong correlation between NEIALs and dynamic rayed aurora as well as red background emissions, which showed that there is a significant part of low energy electrons among the precipitating electrons. We use optical observations to estimate the electron precipitation characteristics of NEIAL events observed with the ESR during nighttime from Svalbard, with particular emphasis on the 844.6 nm emission line
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