Articles | Volume 22, issue 7
Ann. Geophys., 22, 2497–2506, 2004

Special issue: Spatio-temporal analysis and multipoint measurements in space...

Ann. Geophys., 22, 2497–2506, 2004

  14 Jul 2004

14 Jul 2004

The structure of high altitude O+ energization and outflow: a case study

H. Nilsson1, S. Joko1, R. Lundin1, H. Rème2, J.-A. Sauvaud2, I. Dandouras2, A. Balogh3, C. Carr3, L. M. Kistler4, B. Klecker5, C. W. Carlson6, M. B. Bavassano-Cattaneo7, and A. Korth8 H. Nilsson et al.
  • 1Swedish Institute of Space Physics, Kiruna, Sweden
  • 2Centre d’Etude Spatiale des Rayonnements, Toulouse, France
  • 3Imperial College of Science, Technology and Medicine, London, United Kingdom
  • 4University of New Hampshire, Durham, USA
  • 5Max-Planck-Institut für Extraterrestriche Physik, Garching, Germany
  • 6Space Science Laboratory, University of California, Berkeley, USA
  • 7Instituto di Fisica dello Spazio Interplanetario, Roma, Italy
  • 8Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany

Abstract. Multi-spacecraft observations from the CIS ion spectrometers on board the Cluster spacecraft have been used to study the structure of high-altitude oxygen ion energization and outflow. A case study taken from 12 April 2004 is discussed in more detail. In this case the spacecraft crossed the polar cap, mantle and high-altitude cusp region at altitudes between 4RE and 8RE and 2 of the spacecraft provided data. The oxygen ions were seen as a beam with narrow energy distribution, and increasing field-aligned velocity and temperature at higher altitude further in the upstream flow direction. The peak O+ energy was typically just above the highest energy of observed protons. The observed energies reached the upper limit of the CIS ion spectrometer, i.e. 38keV. Moment data from the spacecraft have been cross-correlated to determine cross-correlation coefficients, as well as the phase delay between the spacecraft. Structures in ion density, temperature and field-aligned flow appear to drift with the observed field-perpendicular drift. This, together with a velocity dispersion analysis, indicates that much of the structure can be explained by transverse heating well below the spacecraft. However, temperature isotropy and the particle flux as a function of field-aligned velocity are inconsistent with a single altitude Maxwellian source. Heating over extended altitude intervals, possibly all the way up to the observation point, seem consistent with the observations.