ANGEO Annales Geophysicae ANGEO 1432-0576 Copernicus GmbH Göttingen, Germany 10.5194/angeo-19-1655-2001 Cluster boundary layer measurements and optical observations at magnetically conjugate sites Moen J. 1 2 Holtet J. A. 1 Pedersen A. 1 Lybekk B. 1 Svenes K. 3 Oksavik K. 4 Denig W. F. 5 Lucek E. 6 Søraas F. 4 André M. 7 Department of Physics, University of Oslo, P. O. Box 1048 Blindern, N-0316 Oslo, Norway Also at Arctic Geophysics, University Courses on Svalbard, N-9170 Longyearbyen, Norway Norwegian Defence Research Establishment, Division for Electronics, P. O. Box 25, N-2007 Kjeller, Norway Department of Physics, University of Bergen, N-5007 Bergen, Norway Air Force Research Laboratory, Space Vehicles Directorate, Hanscom AFB, MA 01731–3010, USA Space and Atmospheric Physics Group, Blackett Laboratory, Imperial College, London, SW7 2BW, UK Swedish Institute of Space Physics, Uppsala Division P. O. Box 537, SE-751 21 Uppsala, Sweden 30 11 1999 19 10/12 1655 1668 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.ann-geophys.net/19/1655/2001/angeo-19-1655-2001.html The full text article is available as a PDF file from http://www.ann-geophys.net/19/1655/2001/angeo-19-1655-2001.pdf

The Cluster spacecraft experienced several boundary layer encounters when flying outbound from the magnetosphere to the magnetosheath in the dusk sector on 14 January 2001. The dayside boundary layer was populated by magnetosheath electrons, but not with quite as high densities as in the magnetosheath itself. The Cluster ground track was calculated using the Tsyganenko-96 model which appears to be a strong tool for combining high-altitude satellite and ground observations, given that the solar wind conditions are known. This paper focuses on identifying auroral responses corresponding to boundary layer dynamics observed by Cluster. The first boundary layer encounter studied was a brief visit into a closed LLBL, most likely due to a boundary wave that travelled tailward over the spacecraft. A corresponding equatorward and eastward movement was seen in the post-noon aurora between Greenland and Svalbard. The second boundary encounter was in a high-latitude cusp, and occurred as a consequence of a transient reconfiguration of the cusp. The cusp expanded duskward over the spacecraft into the late post-noon sector. NOAA-12 probed the 16:30 MLT sector of this auroral activity, and measured a 1.4 keV electron beam located poleward of the 30 keV electron-trapping boundary. A sequence of three moving auroral forms emanating from this active region are likely candidates for flux transfer events. The auroral signatures are discussed in relation to earlier observations, and appear to be an example of accelerated electrons/discrete post-noon aurora on open magnetic field lines.<br><br><b>Key words. </b>Ionosphere (particle precipitation) Magnetospheric physics (auroral phenomena; magnetopause, cusp and boundary layers)