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Structure of plasmaspheric plumes and their participation in magnetopause reconnection first results from THEMIS J P McFadden1 C W Carlson1 D Larson1 J Bonnell1 F S Mozer1 V Angelopoulos1 2 K H Glassmeier3 U Auster3 1 Space Sciences Laboratory University of California Berkeley 2 IGPP UCLA Los Angeles CA 90095 3 TUBS Germany Abstract New observations by the THEMIS spacecraft have revealed dense 10 cm 3 plasmaspheric plumes extending to the magnetopause The large scale radial structure of these plumes is revealed by multi spacecraft measurements Temporal variations in the radial distribution of plume plasma caused by azimuthal density gradients coupled with azimuthal flow are also shown to contribute to plume structure In addition flux tubes with cold plume plasma are shown to participate in reconnection with simultaneous observations of cold ions and reconnection flow jets on open flux tubes as revealed by the loss of hot magnetospheric electrons 1 1 Introduction The relative importance of solar wind and ionospheric input to the magnetosphere s plasma has been a subject of study and debate for decades The cold plasma content of the magnetosphere is often ignored due to difficulties in its measurement If we neglect the distant magnetotail 100 Re then cold plasma clearly dominates the total plasma content of the magnetosphere with the plasmasphere often containing an order of magnitude more plasma than the 100 Re magnetotail As demonstrated below dusk side plasmaspheric plumes generally contain 10 100 times more cold plasma than hot plasma In this paper we present THEMIS observations of cold plasma in the near equatorial dayside magnetosphere illustrating the important role cold plasma plays in the dayside magnetosphere including reconnection dynamics THEMIS is not the first satellite to measure cold plasma in the dayside magnetosphere Plasmaspheric plumes were sampled and characterized at low altitudes by the Dynamics Explorer spacecraft Craven et al 1997 The eruption

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