Energy and flux variations across thin auroral arcs

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<sup>−1</sup>) 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

Electrodynamics and energy characteristics of aurora at high resolution by optical methods

“…Lanchester et al [] demonstrated the strength of the method by determining the peak energy and flux of the precipitation forming a number of dynamic auroral events with widths of the order of 100 m perpendicular to the geomagnetic field. Dahlgren et al [] used the technique in an investigation of the role of sheared aurora at boundary regions. In the study, it was found that a thin, sheared auroral arc appeared on the boundary between two different precipitating electron populations of different energies, where an energy gradient and increased number flux were present.…”

Investigation of energy transport and thermospheric upwelling during quiet magnetospheric and ionospheric conditions from the studies of low- and middle-altitude cusp

“…The appearance of optical aurora often suggests also the presence of small-scale, less than a kilometre wide FACs. These have indeed been found with the help of sufficiently rapidly sampling magnetometers on board satellites in low orbits -for example, see Dahlgren et al (2011). The Ørst-edt and CHAMP satellites, launched in 1999 and 2001, respectively, featured high-precision and fast sampling (10 and 50 Hz) fluxgate magnetometers as well as polar orbits.…”

Results from the intercalibration of optical low light calibration sources 2011

“…), optical instruments are usually absolute calibrated by exposing the instrument to a calibration light source with a known spectral radiant sterance corresponding to a certain column emission rate (see, for example, Trondsen, 1998;Mäkinen, 2001;Brändström, 2003, and references therein). Instead of using calibration light sources, some instruments are calibrated by using known spectra of stars (for example Dahlgren et al, 2011).…”