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Ann. Geophys., 24, 1665-1684, 2006
www.ann-geophys.net/24/1665/2006/
© European Geosciences Union 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
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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