| Summary: | In this thesis, we present Naturally Enhanced Ion Acoustic Lines (NEIALs) observed with the EISCAT Svalbard Radar and a comparison with auroral emissions lines measured with a Meridian Scanning Photometer. NEIALs are short-lived events noticeable in the incoherent scatter spectra as an enhancement in one or both shoulders. It has been assumed that NEIALs are associated with an active red emission line in the aurora with an intensity above 10 kR. The red emission line intensity is associated with soft particle precipitation. However, NEIALs have also been observed at low altitudes where particles with higher energies are present, and they are also associated with a severe geomagnetic activity (>500 nT). In the literature, there are three competing theories which might explain the cause of NEIALs in the ionosphere, and they all require free energy provided by auroral particle precipitation. The theories are associated with partly different energy ranges, and auroral emission lines may give an indication of the energies of the precipitating particles. We analyzed four separate days where there were NEIAL incidents. For most of the events, the red line intensity was greater than 10 kR. This agrees with previous observations, and it supports the parametric decay of Langmuir wave theory, which requires soft particles precipitation. However, we have identified 7 events where the red line intensities were below 10 kR, and this weakens the requirement of 10 kR for NEIALs to appear. When NEIALs were observed at lower altitudes (<400 km), the intensity of the green line increased and the right shoulder in the spectra was the most enhanced. This could suggest that at these altitudes, the current driven instability might be the prevailing theory for the production of NEIALs. We also found that increased power in the incoherent scatter spectra might be associated with enhancements in the red line intensity. NEIALs were also observed when the geomagnetic activity was below 500 nT.
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