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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 25, issue 4
Ann. Geophys., 25, 953–969, 2007
© Author(s) 2007. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 25, 953–969, 2007
© Author(s) 2007. This work is distributed under
the Creative Commons Attribution 3.0 License.

  08 May 2007

08 May 2007

CLUSTER observations of electron outflowing beams carrying downward currents above the polar cap by northward IMF

A. Teste1, D. Fontaine1, J.-A. Sauvaud2, R. Maggiolo2, P. Canu1, and A. Fazakerley3 A. Teste et al.
  • 1CETP, 10–12 avenue de l'Europe, Vélizy 78140, France
  • 2CESR, 9 avenue du Colonel Roche, Toulouse 31028, France
  • 3MSSL, Holmbury St Mary, Dorking RH5 6NT, UK

Abstract. Above the polar cap, at about 5–9 Earth radii (RE) altitude, the PEACE experiment onboard CLUSTER detected, for the first time, electron beams outflowing from the ionosphere with large and variable energy fluxes, well collimated along the magnetic field lines. All these events occurred during periods of northward or weak interplanetary magnetic field (IMF).

These outflowing beams were generally detected below 100 eV and typically between 40 and 70 eV, just above the photoelectron level. Their energy gain can be explained by the presence of a field-aligned potential drop below the spacecraft, as in the auroral zone. The careful analysis of the beams distribution function indicates that they were not only accelerated but also heated. The parallel heating is estimated to about 2 to 20 eV and it globally tends to increase with the acceleration energy. Moreover, WHISPER observed broadband electrostatic emissions around a few kHz correlated with the outflowing electron beams, which suggests beam-plasma interactions capable of triggering plasma instabilities.

In presence of simultaneous very weak ion fluxes, the outflowing electron beams are the main carriers of downward field-aligned currents estimated to about 10 nA/m2. These electron beams are actually not isolated but surrounded by wider structures of ion outflows. All along its polar cap crossings, Cluster observed successive electron and ion outflows. This implies that the polar ionosphere represents a significant source of cold plasma for the magnetosphere during northward or weak IMF conditions. The successive ion and electron outflows finally result in a filamented current system of opposite polarities which connects the polar ionosphere to distant regions of the magnetosphere.

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