Abstract
Abstract. Above the polar caps and during prolonged periods of northward IMF, the Cluster satellites detect upward accelerated ion beams with energies up to a few keV. They are associated with converging electric field structures indicating that the acceleration is caused by a quasi-static field-aligned electric field that can extend to altitudes higher than 7 RE (Maggiolo et al., 2006; Teste et al., 2007). Using the AMDA science analysis service provided by the Centre de Données de la Physique des Plasmas, we have been able to extract about 200 events of accelerated upgoing ion beams above the polar caps from the Cluster database. Most of these observations are taken at altitudes lower than 7 RE and in the Northern Hemisphere. We investigate the statistical properties of these ion beams. We analyze their geometry, the properties of the plasma populations and of the electric field inside and around the beams, as well as their dependence on solar wind and IMF conditions. We show that ~40 % of the ion beams are collocated with a relatively hot and isotropic plasma population. The density and temperature of the isotropic population are highly variable but suggest that this plasma originates from the plasma sheet. The ion beam properties do not change significantly when the isotropic, hot background population is present. Furthermore, during one single polar cap crossing by Cluster it is possible to detect upgoing ion beams both with and without an accompanying isotropic component. The analysis of the variation of the IMF BZ component prior to the detection of the beams indicates that the delay between a northward/southward turning of IMF and the appearance/disappearance of the beams is respectively ~2 h and 20 min. The observed electrodynamic characteristics of high altitude polar cap ion beams suggest that they are closely connected to polar cap auroral arcs. We discuss the implications of these Cluster observations above the polar cap on the magnetospheric dynamics and configuration during prolonged periods of northward IMF.
Highlights
Field-aligned acceleration by quasi-static electric fields has been widely reported in the auroral zone (e.g. Mozer et al, 1977; Reiff et al, 1988; Ergun et al, 1998)
As polar cap ion beams (PCIB) are observed during prolonged periods of northward interplanetary magnetic field (IMF), we suggest that the Cold Dense Plasma Sheet (CDPS) may be the source of the background population
In this paper we have discussed Cluster observations of ion beams with an inverted-V shape detected above the polar caps, at altitudes comprised between ∼3 and 7.8 RE
Summary
Field-aligned acceleration by quasi-static electric fields has been widely reported in the auroral zone (e.g. Mozer et al, 1977; Reiff et al, 1988; Ergun et al, 1998). At high altitude and above the polar caps, outflowing ion beams with inverted-V structures have been observed by Cluster at altitudes ranging from 4 to 8 RE during prolonged periods of northward IMF (Maggiolo et al 2006). These ion beams are locally accelerated by quasi-static electric field structures with a field-aligned electric field extending to higher altitude (up to 5 RE) than the parallel E-field of the auroral zone. This section focuses on the properties of the ion populations detected inside the acceleration structures, on the orientation and size of the beams, and on the estimation of the total ion outflow associated with PCIB
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