ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-29-1699-2011 Energy and flux variations across thin auroral arcs Dahlgren H. 1 5 Gustavsson B. 2 Lanchester B. S. 2 Ivchenko N. 1 Brändström U. 3 Whiter D. K. 2 4 Sergienko T. 3 Sandahl I. 3 Marklund G. 1 Space and Plasma Physics, School of Electrical Engineering, KTH, Stockholm, Sweden School of Physics and Astronomy, University of Southampton, UK Swedish Institute of Space Physics, Kiruna, Sweden Finnish Meteorological Institute, Helsinki, Finland now at: Dept. of Electrical and Computer Engineering and Center for Space Physics, Boston University, USA 04 10 2011 29 10 1699 1712 Copyright: © 2011 H. Dahlgren et al. 2011 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/29/1699/2011/angeo-29-1699-2011.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/29/1699/2011/angeo-29-1699-2011.pdf

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.

 References Aso, T., Ejiri, M., Urashima, A., Miyaoka, H., Steen, Å., Br{ä}ndstr{ö}m, U., and Gustavsson, B.: First results of auroral tomography from ALIS-Japan multi-station observations in March, 1995, Earth, Planets, and Space, 50, 81–86, 1998. Baker, D.&nbsp;J. and Romick, G.&nbsp;J.: The rayleigh: interpretation of the unit in terms of column emission rate or apparent radiance expressed in SI units, Appl. Optics, 15, 1966–1968, 1976. Borovsky, J.&nbsp;E., Suszcynsky, D.&nbsp;M., Buchwald, M.&nbsp;I., and DeHaven, H.&nbsp;V.: Measuring the thicknesses of auroral curtains, Arctic, 44, 231–238, 1991. Br{ä}ndstr{ö}m, U.: The Auroral Large Imaging System - design, operation and scientific results, IRF Scientific Report 279, ISBN: 91-7305-405-4, PhD thesis, 2003. Burnham, K.&nbsp;P. and Anderson, D.: Model Selection and Multimodel Inference: A Practical Information-Theoretic Approach, Springer-Verlag, 2 edn., 2002. Chaston, C.&nbsp;C., Peticolas, L.&nbsp;M., Bonnell, J.&nbsp;W., Carlson, C.&nbsp;W., Ergun, R.&nbsp;E., McFadden, J.&nbsp;P., and Strangeway, R.&nbsp;J.: Width and brightness of auroral arcs driven by inertial Alfven waves, J. Geophys. Res., 108, <a href="http://dx.doi.org/10.1029/2001JA007537">https://doi.org/10.1029/2001JA007537</a>, 2003. Chaston, C.&nbsp;C., Seki, K., Sakanoi, T., Asamura, K., and Hirahara, M.: Motion of aurorae, Geophys. Res. Lett., 37, L08104, <a href="http://dx.doi.org/10.1029/2009GL042117">https://doi.org/10.1029/2009GL042117</a>, 2010. Dahlgren, H., Ivchenko, N., Sullivan, J., Lanchester, B. S., Marklund, G., and Whiter, D.: Morphology and dynamics of aurora at fine scale: first results from the ASK instrument, Ann. Geophys., 26, 1041–1048, <a href="http://dx.doi.org/10.5194/angeo-26-1041-2008">https://doi.org/10.5194/angeo-26-1041-2008</a>, 2008. Dahlgren, H., Ivchenko, N., Lanchester, B.&nbsp;S., Ashrafi, M., Whiter, D., Marklund, G., and Sullivan, J.: First direct optical observations of plasma flows using afterglow of O<sup>+</sup> in discrete aurora, J.&nbsp;Atmos.&nbsp;Solar-Terr.&nbsp;Phys., 71, 228–238, <a href="http://dx.doi.org/10.1016/j.jastp.2008.11.015">https://doi.org/10.1016/j.jastp.2008.11.015</a>, 2009. Dahlgren, H., Aikio, A., Kaila, K., Ivchenko, N., Lanchester, B.&nbsp;S., Whiter, D., and Marklund, G.: First direct optical observations of plasma flows using afterglow of O<sup>+</sup> in discrete aurora, J.&nbsp;Atmos.&nbsp;Solar-Terr.&nbsp;Phys., 72, 633–637, <a href="http://dx.doi.org/10.1016/j.jastp.2010.01.014">https://doi.org/10.1016/j.jastp.2010.01.014</a>, 2010. Frey, H.&nbsp;U., Haerendel, G., Clemmons, J.&nbsp;H., Bochm, M.&nbsp;H., Vogt, J., Bauer, O.&nbsp;H., Wallis, D.&nbsp;D., Blomberg, L., and L{ü}hr, H.: Freja and ground-based analysis of inverted-V events, J. Geophys. Res., 103, 4303–4314, <a href="http://dx.doi.org/10.1029/97JA02259">https://doi.org/10.1029/97JA02259</a>, 1998. Gordon, R. and Herman, G.&nbsp;T.: Three-dimensional reconstruction from projections; a review of algorithms, Int. Rev. Cytol., 38, 111–151, 1974. Gustavsson, B.: Tomographic inversion for ALIS noise and resolution, J. Geophys. Res., 103, 26621–26632, 1998. Gustavsson, B.: Three dimensional imaging of aurora and airglow, PhD thesis, 2000. Gustavsson, B., Steen, Å., Sergienko, T., and Br{ä}ndstr{ö}m, B. U.&nbsp;E.: Estimate of auroral electron spectra, the power of ground-based multi-station optical measurements, Phys. Chem. Earth, 26, 189–194, <a href="http://dx.doi.org/10.1016/S1464-1917(00)00106-9">https://doi.org/10.1016/S1464-1917(00)00106-9</a>, 2001. Hecht, J.&nbsp;H., Christensen, A.&nbsp;B., Strickland, D.&nbsp;J., and Meier, R.&nbsp;R.: Deducing composition and incident electron spectra from ground-based auroral optical measurements: Variation in oxygen density, J. Geophys. Res., 94, 13553–13563, 1989. Hunten, D.&nbsp;M., Roach, F.&nbsp;E., and Chamberlain, J.&nbsp;W.: A photometric unit for the airglow and aurora, JATP, 8, 345–346, 1956. Ivchenko, N., Blixt, E.&nbsp;M., and Lanchester, B.&nbsp;S.: Multispectral observations of auroral rays and curls, Geophys. Res. Lett., 32, L18106, <a href="http://dx.doi.org/10.1029/2005GL022650">https://doi.org/10.1029/2005GL022650</a>, 2005. Johansson, T., Marklund, G., Karlsson, T., Liléo, S., Lindqvist, P.-A., Marchaudon, A., Nilsson, H., and Fazakerley, A.: On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries, Ann. Geophys., 24, 1713–1723, http://dx.doi.org/10.5194/angeo-24-1713-2006https://doi.org/10.5194/angeo-24-1713-2006, 2006. Lanchester, B.&nbsp;S., Palmer, J.&nbsp;R., Rees, M.&nbsp;H., Lummerzheim, D., Kaila, K., and Turunen, T.: Energy flux and characteristic energy of an elemental auroral structure, Geophys. Res. Lett., 21, 2789–2792, 1994. Lanchester, B.&nbsp;S., Kaila, K., and McCrea, I.&nbsp;W.: Relationship between large horizontal electric fields and auroral arc elements, J. Geophys. Res., 101, 5075–5084, <a href="http://dx.doi.org/10.1029/95JA02055">https://doi.org/10.1029/95JA02055</a>, 1996. Lanchester, B.&nbsp;S., Rees, M.&nbsp;H., Lummerzheim, D., Otto, A., Frey, H.&nbsp;U., and Kaila, K.&nbsp;U.: Large fluxes of auroral electrons in filaments of 100 m width, J. Geophys. Res., 102, 9741–9748, 1997. Lanchester, B. S., Rees, M. H., Sedgemore, K. J. F., Palmer, J. R., Frey, H. U., and Kaila, K. U.: Ionospheric response to variable electric fields in small-scale auroral structures, Ann. Geophys., 16, 1343–1354, <a href="http://dx.doi.org/10.1007/s00585-998-1343-8">https://doi.org/10.1007/s00585-998-1343-8</a>, 1998. Maggs, J.&nbsp;E. and Davis, T.&nbsp;N.: Measurements of the thicknesses of auroral structures, Planet. Space Sci., 16, 205–209, 1968. Meier, R.&nbsp;R., Strickland, D.&nbsp;J., Hecht, J.&nbsp;H., and Christensen, A.&nbsp;B.: Deducing composition and incident electron spectra from ground-based auroral optical measurements: A study of auroral red line processes, J. Geophys. Res., 94, 13541–13552, 1989. Palmer, J.&nbsp;R.: Plasma density variations in aurora, PhD thesis, 1995. Rees, M.: Physics and chemistry of the upper atmosphere, Cambridge University Press, 1989. Roth, M., Evans, D.&nbsp;S., and Lemaire, J.: Theoretical structure of a magnetospheric plasma boundary – Application to the formation of discrete auroral arcs, J. Geophys. Res., 98, 11411–11423, <a href="http://dx.doi.org/10.1029/93JA00156">https://doi.org/10.1029/93JA00156</a>, 1993. Sandahl, I., Sergienko, T., and Br{ä}ndstr{ö}m, U.: Fine structure of optical aurora, J.&nbsp;Atmos.&nbsp;Solar-Terr.&nbsp;Phys., 70, 2275–2292, 2008. Semeter, J. and Kamalabadi, F.: Determination of primary electron spectra from incoherent scatter radar measurements of the auroral E region, Radio Sci., 40, RS2006, <a href="http://dx.doi.org/10.1029/2004RS003042">https://doi.org/10.1029/2004RS003042</a>, 2005. Semeter, J., Lummerzheim, D., and Haerendel, G.: Simultaneous multispectral imaging of the discrete aurora, J.&nbsp;Atmos.&nbsp;Solar-Terr.&nbsp;Phys., 63, 1981–1992, 2001. Semeter, J., Zettergren, M., Diaz, M., and Mende, S.: Wave dispersion and the discrete aurora: New constraints derived from high-speed imagery, J. Geophys. Res., 113, A12208, <a href="http://dx.doi.org/10.1029/2008JA013122">https://doi.org/10.1029/2008JA013122</a>, 2008. Sergienko, T.&nbsp;I. and Ivanov, V.&nbsp;E.: A new approach to calculate the excitation of atmospheric gases by auroral electron impact, Ann.&nbsp;Geophys., 11, 717–727, 1993. Sergienko, T., Sandahl, I., Gustavsson, B., Andersson, L., Brändström, U., and Steen, Å.: A study of fine structure of diffuse aurora with ALIS-FAST measurements, Ann. Geophys., 26, 3185–3195, <a href="http://dx.doi.org/10.5194/angeo-26-3185-2008">https://doi.org/10.5194/angeo-26-3185-2008</a>, 2008. Stasiewicz, K., Bellan, P., Chaston, C., Kletzing, C., Lysak, R., Maggs, J., Pokhotelov, O., Seyler, C., Shukla, P., Stenflo, L., Streltsov, A., and Wahlund, J.-E.: Small scale Alfv{é}nic structure in the aurora, Space Sci. Rev., 92, 423–533, 2000. Strickland, D.&nbsp;J., Meier, R.&nbsp;R., Hecht, J.&nbsp;H., and Christensen, A.&nbsp;B.: Deducing composition and incident electron spectra from ground-based auroral optical measurements. I – Theory and model results. II - A study of auroral red line processes. III – Variations in oxygen density, J. Geophys. Res., 94, 13527–13539, 1989. Trondsen, T.&nbsp;S. and Cogger, L.&nbsp;L.: Fine-scale optical observations of aurora, Phys. Chem. Earth, 26, 179–188, 2001. Whiter, D.&nbsp;K., Lanchester, B.&nbsp;S., Gustavsson, B., Ivchenko, N., and Dahlgren, H.: Using multispectral optical observations to identify the acceleration mechanism responsible for flickering aurora, J. Geophys. Res., 115, A12315, <a href="http://dx.doi.org/10.1029/2010JA015805">https://doi.org/10.1029/2010JA015805</a>, 2010.