R. Maggiolo 1 , M. Echim 1,2 , D. Fontaine 3 , A. Teste 4 , C. Jacquey 5 maggiolo@aeronomie.be

1. e- B e- E e- E H+O+ ~5 RE Above the polar caps and at altitudes higher than 4 RE, the Cluster spacecraft detect accelerated ionospheric ion beams with energies up to a few keV. These beams can be separated into 2 categories: 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 the encounter of field aligned proton and oxygen ions with energies differing by a factor of about 4. Polar cap local ions beams are characterized by nearly monoenergetic ion inverted V and are in many occasions associated with a converging electric field. 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. After a description of the method used for the automatic detection of the beams, we present preliminary results about the statistical behavior of these structures. We discuss the local properties of these beams (energy, location…) together with their dependence on the interplanetary conditions (solar wind pressure, IMF orientation and magnitude). E r r ò = - E V . dl D trajectory BYgsm>0 BYgsm<0 BXgsm>0 BXgsm<0 Cluster observation of locally accelerated outflowing ion beams above the polar cap: a statistical study R. Maggiolo1, M. Echim1,2, D. Fontaine3, A. Teste4, C. Jacquey5 maggiolo@aeronomie.be 1 Belgian Institute for Space Aeronomy (IASB-BIRA); 2 Institute for Space Sciences, Bucharest, Romania; 3 Laboratoire de Physique des Plasmas, Velizy, France; 4 Space Sciences Laboratory, Berkeley, United States; 5 Centre d’Etude Spatiale des Rayonnements, Toulouse, France Cluster observation of locally accelerated ion beams above the polar caps---------------------- (Maggiolo et al., Ann. Geophys., 2006; Teste et al., Ann. Geophys.,2007) • During periods of northward IMF and inside the polar cap, Cluster detects structures of ionospheric ions accelerated upward associated with electrons accelerated downward. • These structures are associated with a converging electric field. • The ion energy of the spectra maximum well fit the potential computed from the EDI electric field. • Upward accelerated ions and downward accelerated electrons are simultaneously detected. • The total potential drop of about 1 kV is distributed along the magnetic field lines over distances larger than the spacecraft altitude of 5 RE. • These structures are associated with upward current sheets. • Between the structures upward electron beams with energies up to 50-100 eV are measured. These beams are associated with downward current sheets. Ions Electrons r Automatic detection of ion outflows above the polar caps with AMDA----------------------------- AMDA (http://cdpp.cesr.fr ) is a tool performing automated search of events, producing time-tables. This tool scans the content of the data and automatically test mathematical criteria edited by the user. We applied it to the detection of locally accelerated ion beams. Here is an example describing the method we used. • 2nd condition: above the polar caps (i.e. in the lobe region) • B N>0.05 cm-³ • Extension of B • B’ =B2 min • Intersection: outflow above the polar cap • A ∩ B’ C • 1st condition: ion outflows • A Vpar > 70 km/s et N>0.05 cm-³ B’ N>0.05 cm-³ A Vpar>70 km/s B B’ C A No condition on Cluster orbit. The lobe region is identified by its low plasma density N>0.05 cm-3 and Vpar> 70 km/s The ion beams are detected excluded Preliminary results---------------------------------------------------------------------------------------------------- Between 2001 and 2006, around 1500 time periods have been selected with AMDA. A visual inspection of these time periods is ongoing. About 20% of these time periods actually correspond to local ion beams. Preliminary results obtained with a first set of ~50 events are presented below. Not measured IMF The occurrence of these arcs doesn’t depend on the solar wind pressure and velocity. It also doesn’t seem to depend on IMF BX and BY components. However, it requires prolonged periods of northward IMF before it appears. There should be a delay between the appearance of the beams and a northward turning of IMF. All the events occurs during periods of very low geomagnetic activity. A significant number of events are detected while the IMF is oriented southward. There should be a delay between the disappearance of the beams and a southward turning of IMF. Vsw BXgsm The ion energy is typically lower than 1 keV with a maximum around 300 eV Average 2001-2006 BYgsm Psw Average 2001-2006 BZgsm BZgsm This “inertia” suggests that these beams are linked to the large scale configuration of the magnetosphere during periods of northward IMF. Kp Location of the beams detected above the northern hemisphere Bx >0 (<0) seems to favor beams in the duskside (dawnside). We need more events to confirm this tendency.