Abstract
Abstract. Simultaneous images of the aurora in three emissions, N21P (673.0 nm), OII (732.0 nm) and OI (777.4 nm), have been analysed; the ratio of atomic oxygen to molecular nitrogen has been used to provide estimates of the changes in energy and flux of precipitation within scale sizes of 100 m, and with temporal resolution of 32 frames per second. The choice of filters for the imagers is discussed, with particular emphasis on the choice of the atomic oxygen line at 777.4 nm as one of the three emissions measured. The optical measurements have been combined with radar measurements and compared with the results of an auroral model, hence showing that the ratio of emission rates OI/N2 can be used to estimate the energy within the smallest auroral structures. In the event chosen, measurements were made from mainland Norway, near Troms\\o, (69.6 N, 19.2 E). The peak energies of precipitation were between 1–15 keV. In a narrow curling arc, it was found that the arc filaments resulted from energies in excess of 10 keV and fluxes of approximately 7 mW/m2. These filaments of the order of 100 m in width were embedded in a region of lower energies (about 5–10 keV) and fluxes of about 3 mW/m2. The modelling results show that the method promises to be most powerful for detecting low energy precipitation, more prevalent at the higher latitudes of Svalbard where the multispectral imager, known as ASK, is now installed.
Highlights
IntroductionThe highest spatial and temporal resolution images of aurora show that there is a large variety of auroral fine-scale structure (tens of metres across the magnetic field) which is highly dynamic
The highest spatial and temporal resolution images of aurora show that there is a large variety of auroral fine-scale structure which is highly dynamic
Low energy electrons give rise to atomic oxygen emissions at heights of 200–500 km, while high energy electrons reach down to approximately 100 km and give a spectrum dominated by molecular emissions
Summary
The highest spatial and temporal resolution images of aurora show that there is a large variety of auroral fine-scale structure (tens of metres across the magnetic field) which is highly dynamic. Processes operating on small scales often affect the bulk properties of the plasma, and so characterising and understanding the structuring is important. In aurora, this structuring can be directly observed in the distribution of emitted light. The combination of a number of imagers operating in different spectral lines allows timedependent, two-dimensional energy spectrum maps of the auroral precipitation to be generated. This is the best available “image” of the acceleration processes. In situ spacecraft provide more accurate information on particle distribution functions, electric fields and plasma densities than remote sensing, but they lack the detailed two-dimensional structure and rapid evolution of optical measurements
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