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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 toprovide estimates of the changes in energy and flux ofprecipitation within scale sizes of 100 m, and with temporalresolution of 32 frames per second. The choice of filters for theimagers is discussed, with particular emphasis on the choice ofthe atomic oxygen line at 777.4 nm as one of the three emissionsmeasured. The optical measurements have been combined with radarmeasurements and compared with the results of an auroral model,hence showing that the ratio of emission rates OI/N2 can beused to estimate the energy within the smallest auroralstructures. In the event chosen, measurements were made frommainland Norway, near Tromso, (69.6 N, 19.2 E). The peak energiesof precipitation were between 1–15 keV. In a narrow curling arc,it was found that the arc filaments resulted from energies inexcess 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 about3 mW/m2. The modelling results show that the methodpromises to be most powerful for detecting low energyprecipitation, more prevalent at the higher latitudes of Svalbardwhere the multispectral imager, known as ASK, is now installed

annales geophysicae

Simultaneous imaging of aurora on small scale in OI (777.4 nm) and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;1P to estimate energy and flux of precipitation