Abstract
Abstract. Two discrete auroral arc filaments, with widths of less than 1 km, have been analysed using multi-station, multi-monochromatic optical observations from small and medium field-of-view imagers and the EISCAT radar. The energy and flux of the precipitating electrons, volume emission rates and local electric fields in the ionosphere have been determined at high temporal (up to 30 Hz) and spatial (down to tens of metres) resolution. A new time-dependent inversion model is used to derive energy spectra from EISCAT electron density profiles. The energy and flux are also derived independently from optical emissions combined with ion-chemistry modelling, and a good agreement is found. A robust method to obtain detailed 2-D maps of the average energy and number flux of small scale aurora is presented. The arcs are stretched in the north-south direction, and the lowest energies are found on the western, leading edges of the arcs. The large ionospheric electric fields (250 mV m−1) found from tristatic radar measurements are evidence of strong currents associated with the region close to the optical arcs. The different data sets indicate that the arcs appear on the boundaries between regions with different average energy of diffuse precipitation, caused by pitch-angle scattering. The two thin arcs on these boundaries are found to be related to an increase in number flux (and thus increased energy flux) without an increase in energy.
Highlights
High resolution imaging of aurora borealis from the ground has revealed the inherent occurrence of small scale structuring, with auroral filaments of widths of just a few tens of metres appearing on time scales of a fraction of a second (Maggs and Davis, 1968; Borovsky et al, 1991; Trondsen and Cogger, 2001)
The EISCAT measurements provide an independent method to estimate the average energy of the precipitation
The strength of combined optical and radar observations has been demonstrated for an auroral event on 12 December 2006
Summary
High resolution imaging of aurora borealis from the ground has revealed the inherent occurrence of small scale structuring, with auroral filaments of widths of just a few tens of metres appearing on time scales of a fraction of a second (Maggs and Davis, 1968; Borovsky et al, 1991; Trondsen and Cogger, 2001). ) and from e(aIalOisg)tn,-weadnedcstuthtaselittghonrpoekudegochgutrthasemtihmirsoaaugcgeosh.lotIhuOer-imcimoadgaegedse(sfr.oerIdOd-gifirfmeeeraneg-nbetlstuifemo)erosdvaeifrrfeleasryheeondwtktneimobgeerlsaomwareothfsethhooevwethrnlraebyeeecldohwaknentohegelrsao.mvA.elrllkaeyoegdrakmeosgarraemm.ade from east-west torohefngeetihrociingas,nhawtreichxnipclteeohcreItrOfiepsegipsrusomrpneeo)dc.rsteTivteohefefitehwcffeieeidcdntetislfryfwuuenshxesectneritumevcdiitesuwisnriieontdhngesatrhaFcep-trpoaeeugtarhiooreirnnaw,geisnt leEaIvSinCgATbeahninddmaondaeflltienrgglowing O+ emission in ASK2 The afterglow of this event has been analysed in Dahlgren et al. AofSf Kzenointhly(Saefmewetesreectoanl.d,s20b0e8fo).reInthtehe1r9e3g2ioanrca.roTunhde m19a3g2- arc snheotiwc szeenviitdhe, nthcee oaftormayics,osxeyegneanseinmtiesnsisoifincsaeteionnbsyalAoSnKg 3thiesarc. The maximum and minimum ratios and energies for the 1932 and 1933 arcs, estimated across the east-west aligned cut marked in red, are given in Fig. 5 by the red dotted lines and green dashed lines, respectively. As the arc passes through zenith, IO does not remain closer to magnetic zenith, but continues peaking to the west of the IO+2 peak This effect cannot be a product of perspective, and has to be explained by variation in average energy across the arc
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