2008gl0336773. ObservationsAcknowledgementsReferences2008gl033677-p012008gl033677-p022008gl033677-p03Structure 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. 11. 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 and structure of plasmaspheric plumes at geosynchronous orbit during active times has been observed using the MPA instruments on the LANL Geosynchronous satellites [Su et al., 2001]. The participation of cold plasma in reconnection at the magnetopause has also been inferred during times of high solar wind dynamics pressure when the magnetopause was at or inside of geosynchronous orbit [Su et al., 2000]. The Cluster spacecraft measured cold ions near the high latitude magnetopause (MP) when solar wind dynamic pressure changes revealed the cold ions [Sauvaud et al., 2001]. In addition, the IMAGE spacecraft was able to capture the formation and loss of plasmaspheric plumes [Goldstein et al., 2004]. The THEMIS mission provides a much more extensive, detailed, high resolution data set of 2cold plasma observations within plasmaspheric plumes and at the magnetopause during reconnection. In particular these first results demonstrate radial and small scale structure in plasmaspheric plumes, and reveal cold plasma capture during reconnection at the MP during nominal solar wind conditions. 2. THEMIS Mission and Instruments THEMIS is a five satellite mission whose prime science objective is to measure the time sequences of substorm dynamics. The first seven months of the mission consisted of a “coast phase” that kept the satellites in close proximity in preparation for their final injections into the prime-mission orbit configurations (Angelopoulos et al., 2008). This close separation allowed cross-calibration of the instruments and meso-scale studies of the dayside magnetosphere. In particular the spacecraft were oriented in a string-of-pearls configuration whose orbit had apogee (perigee) of ~14.7 Re (~1.16 Re), with the initial line-of-apsides near dusk. The spacecraft are identified by letters and were ordered B-D-C-E-A starting with the leading probe. In the vicinity of the MP, the inner three probes were spaced by ~1000 km and the leading and trailing probes spaced by ~3000 km. This orbit and satellite configuration resulted in spacecraft separations that were aligned within ~35o of the MP normal. The “coast phase” configuration provided one or two passes through the nominal plasmaspheric plume region (12-18 LT) each day for several months, followed by pre-noon observations of cold plasma outside the plume region. Cold plasma is difficult to detect and quantify so we use a combination of three different instruments on THEMIS to make this measurement. An ion plasma sensor detects cold plasma when convection flows overcome the potential barrier caused by spacecraft charging. This generally 3restricts cold ion detection to regions near the MP where solar wind dynamic pressure changes result in plasma motion. Since spacecraft charging generally allows detection of all electrons, comparisons of ion and electron densities can also reveal a missed cold ion component. However the electron measurement is also difficult since cold ions are normally charge-balanced by cold electrons, which are hard to separate from spacecraft photo-electrons. Both plasma sensors are electrostatic analyzers that make 3-D plasma measurements with ~3s resolution. The relative calibration of electron and ion plasma sensors is better than 5%, and the uncertainty in the absolute calibration is 10% [McFadden et al., 2008]. The third method of detecting cold plasma is by monitoring the spacecraft potential, which depends on the plasma density [Pederson et al., 1998, Scudder et al., 2000]. Cross-correlation between measured densities and spacecraft potential, as determined from the EFI Langmuir probes [Bonnell et al., 2008], provides a relation between density and potential. For a given spacecraft geometry, this relation is not constant and depends on the plasma distribution and the bias currents and voltages applied to the Langmuir probes. At the time of this paper, the potential versus density relationship has not been well quantified for all these changes. Instead we use a simple functional relation (3 exponentials) with an adjustable parameter that allows us to tailor the algorithm to obtain a good fit in regions where cold plasma is measured by other instruments. Density inferred by spacecraft potential is especially useful within plasmaspheric plumes far from the MP since the