Radar and optical studies of small scale features in the Aurora: the association of optical signatures with Naturally Enhanced Ion Acoustic Lines (NEIALs)

摘要

Naturally Enhanced Ion Acoustic Lines (NEIALs) are enhancements (often over an order of magnitude in power) observed in ion line spectra in incoherent scatter radar data. To date, there has been no conclusive evidence supporting a single production process which explains all observations of NEIALs. A specially designed interferometric radar system has been used to determine the field line height distribution and field line footpoint of observed NEIALs. This system is the EISCAT Svalbard Radar Aperture Synthesis Imaging experiment, known as EASI. Optical observations were taken with the co-located Auroral Structures and Kinetics facility to compare NEIAL positions to locations of optical phenomena.Results show that the production theories of ‘beam-driven Langmuir waves’ and ‘solitary kinetic Alfv´en waves’ (SKAW) outlined in this thesis, consistently describe observed phenomena during two separate intervals. During the first interval on the 24 January 2012, the ‘non-resonant’ regime of ‘beam-driven Langmuir waves’ dominated, with the ‘resonant’ regime occurring infrequently, throughout approximately half of the interval. The remaining enhancements were only possible through SKAW or other wave interactions. Observations from the second interval on the 27 March 2012 agree best with the ‘resonant’ regime of the Langmuir wave theory. However, a number of time steps may have been enhanced by other production mechanisms, most likely SKAW. Radar-derived parameter analysis of these and four other NEIAL intervals in past data was also undertaken. The results showed the existence of ‘weak’ NEIALs at times surrounding ‘strong’ NEIAL enhancements. Other than the two aforementioned theories, the majority of others were eliminated independently for all six time intervals using this method.