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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 24, issue 6
Ann. Geophys., 24, 1665–1684, 2006
https://doi.org/10.5194/angeo-24-1665-2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 24, 1665–1684, 2006
https://doi.org/10.5194/angeo-24-1665-2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.

  03 Jul 2006

03 Jul 2006

A multi-satellite study of accelerated ionospheric ion beams above the polar cap

R. Maggiolo1, J. A. Sauvaud1, D. Fontaine2, A. Teste2, E. Grigorenko3,4, A. Balogh5, A. Fazakerley6, G. Paschmann7, D. Delcourt8, and H. Rème1 R. Maggiolo et al.
  • 1CESR, 9 avenue du Colonel Roche, Toulouse 31028, France
  • 2CETP, 10 avenue de l’Europe, Vélizy 78140, France
  • 3Institute of Space Research (IKI), Moscow, Russia
  • 4SINP of Moscow State University, Russia
  • 5Space and Atmospheric Physics Group, The Blackett Laboratory, Imperial College, London, UK
  • 6MSSL, Holmbury St Mary, Dorking RH5 6NT, UK
  • 7MPI für extraterrestrische Physik, D-85748 Garching, Germany
  • 8CETP, 4 avenue de Neptune, Saint-Maur-des-Fossés 94107, France

Abstract. This paper presents a study of nearly field-aligned outflowing ion beams observed on the Cluster satellites over the polar cap. Data are taken at geocentric radial distances of the order of 5–9 RE. The distinction is made between ion beams originating from the polar cusp/cleft and beams accelerated almost along the magnetic field line passing by the spacecraft. Polar cusp beams are characterized by nearly field-aligned proton and oxygen ions with an energy ratio EO+ / EH+, of the order of 3 to 4, due to the ion energy repartition inside the source and to the latitudinal extension of the source. Rapid variations in the outflowing ion energy are linked with pulses/modifications of the convection electric field. Cluster data allow one to show that these perturbations of the convection velocity and the associated ion structures propagate at the convection velocity.

In contrast, polar cap local ion beams are characterized by field-aligned proton and oxygen ions with similar energies. These beams show the typical inverted V structures usually observed in the auroral zone and are associated with a quasi-static converging electric field indicative of a field-aligned electric field. The field-aligned potential drop fits well the ion energy profile. The simultaneous observation of precipitating electrons and upflowing ions of similar energies at the Cluster orbit indicates that the spacecraft are crossing the mid-altitude part of the acceleration region. In the polar cap, the parallel electric field can thus extend to altitudes higher than 5 Earth radii. A detailed analysis of the distribution functions shows that the ions are heated during their parallel acceleration and that energy is exchanged between H+ and O+. Furthermore, intense electrostatic waves are observed simultaneously. These observations could be due to an ion-ion two-stream instability.

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