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Ann. Geophys., 25, 953-969, 2007
www.ann-geophys.net/25/953/2007/
© European Geosciences Union 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
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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