Statistical study of ionospheric ion beams observed by CLUSTER above the polar caps

Above the polar caps and during prolonged periods of Northward IMF, the Cluster spacecraft detect accelerated ion beams with energies up to a few keV. They are associated with downward precipitating electrons and converging electric field structures indicating that the acceleration is caused by a quasi-static field aligned electric field that can extend to altitudes up to 5 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 (CDPP, http://cdpp.cesr.fr), we have been able to extract from the Cluster ion detectors dataset the time periods when Cluster encounters polar cap local ion beams. 6 years of data have been mined with this tool. Almost 200 events have been found giving new insight on these structures. After a description of the method used for the automatic detection of the beams, we will discuss their statistical properties. We analyze their relation to solar wind and IMF. In particular, we estimate the delay between a Northward/Southward turning of the IMF and the appearance/disappearance of these beams. The characteristics of the particles detected inside these structures as well as their size, orientation and location are also presented. We show that these ion beams are located on magnetic field lines mapping close to the high latitude magnetopause and in the central part of the lobes and that 40 % of them are detected together with hot isotropic ions. These results will be discussed in term of magnetotail configuration during prolonged periods of Northward IMF.