GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1029/,Multi-point observations of the inner boundary of1the plasma sheet during geomagnetic disturbances2A. Runov,1V. Angelopoulos,1N. Ganushkina,2R. Nakamura,3J.McFadden,4D. Larson,4I. Dandouras,5K.-H. Glassmeier,6C. Carr7A. Runov, Institute o f Geophysics a nd Planetary Physics, University of California, Los Angeles,3845 Slichter Hall, Los Angeles, CA-90095, USA. ([email protected])1Institute of Geophysics and PlanetaryD R A F T April 7, 2008, 11:25am D R A F TX - 2 RUNOV ET AL.: INNER BOUNDARY OF THE PLASMA SHEETWe report on the THEMIS and Double Star TC-1 observations at the geo-3centric distances of 3< R <8 REduring substorms on March 23, 2007.4THEMIS crossed the inner boundary of the equatorial dusk-side plasma in5the string-of-pearls configuration, allowing the dynamics of particle popu-6lations to be traced within time ranges from hours to 10 minutes. Observa-7tions show co-existence of the plasma sheet ion population (5 - 30 keV) with8the ring current ion population (100 - 1000 keV) at 4< R¡6 RE. The plasma9Physics, University of California, LosAngeles, CA-9095, USA2Finnish Meteorological Institute, 11010Helsinki, Finland3Space Research Institute, AustrianAcademy of Sciences, Graz, Austria4Space Science Laboratory, University ofCalifornia, Berkeley, CA, USA5CESR/CNRS, Toulouse, France6Institut f¨ur Geophysik undExtraterrestrische Physik, TechnischeUniversit¨at Bra unschweig, Germany7The Blackett Laboratory, ImperialCollege London SW7 2BW, UKD R A F T April 7, 2008, 11:25am D R A F TRUNOV ET AL.: INNER BOUNDARY OF THE PLASMA SHEET X - 3sheet population was characterized by pronounced “nose”-like dispersion with10the spectral density maximum at ∼10 keV. The plasma sheet boundary, de-11fined by a sharp decrease of the ∼1keV electron flux, moved inward to R=412and outward back to ∼8 RE within about 1 hour. Local enhancements of13the plasma sheet (1 - 5 keV) electron flux with the characteristic time scale14of 2 - 10 min were detected at 5< R¡6 REduring the substorm.15D R A F T April 7, 2008, 11:25am D R A F TX - 4 RUNOV ET AL.: INNER BOUNDARY OF THE PLASMA SHEET1. IntroductionThe magnetotail plasma sheet (PS), containing high-β plasma with the typical ion16energy of 5 - 10 keV and electron energy 1 - 5 keV, is an important source of hot plasma,17observed in the inner magnetosphere. The inner edge of the PS, determined by ∼1 keV18electron flux cutoff [Thomse n et al., 2002] is typically located near geosynchronous orbit19(6.6 RE) [e.g D enton et a l . , 2007]. During active time, PS particles have temporally20limited access to the innermost regions of the magnetosphere [e.g., Ganushkina et al.,212000; Friedel et al., 2001; Denton et al., 2007]. The models including quasi-static global22electric field reproduce observable features such as ion “nose”-like dispersion and ion and23electron convection boundaries [Ejiri et al., 1980 ; Kerns et al., 1994; Angelopoulos et a l . ,242002]. It was found that closest to the Earth ion population may move from geostationary25orbit into the plasmasphere up to L=4.7 in about 1 hour during a substorm [Ganushkina26et al., 2001], while large-scale convection leads to the nose structure formation in > 527hours [Buzulukova et al., 2003]. The implication of more adequate global electric field28models [e.g., Angelopoulos et al., 2002] does not reduce this time sufficiently. The inward29displacement of the intense nose structure can occur under short-lived pulse electric fields30[Li et al., 1998].31Multi-point observations of the inner PS dynamics are required to separate t he effects32of the global and the transient electric fields. Fo ur Cluster spacecraft provided valuable33observations at high latitudes [e.g., Apatenkov et al., 2007], at 4 REat all LT range on the34equator [Vallat et al . , 2007] and simultaneously in opposite MLT sectors in correlation with35Double-Star TC-1 [Dandouras et al., 2 008]. The THEMIS mission provides an opportunity36D R A F T April 7, 2008, 11:25am D R A F TRUNOV ET AL.: INNER BOUNDARY OF THE PLASMA SHEET X - 5to probe the near-equatorial magnetosphere at a wide range of the geocentric distances37[Angelopoulos, 2008]. At the initial phase, five THEMIS probes scanned the dusk sector38of the equatorial magnetosphere in a string-of-pearls configuration, successively crossing39geostationary orbit, allowing to resolve the dynamics of the PS inner boundary during40geomagnetic disturbances with time scales from hours to 10 minutes.41In this letter we report on multi-point probing of ion and electron populations on t he42inner edge of the dusk-side PS during enhanced geomagnetic activity. The aim of this43study is to reveal the temporal evolution of the PS inner edge on substorm time scales in44order to determine the physics necessary to explain or properly model its behavior.452. DataWe discuss THEMIS and TC-1 observations during 11 - 20 UT on March 23, 2007 when46Kpvaried form 0+ to 3+ and AL<-150 nT with two peaks of -300 a nd -700 nT. This47event was intensively studied [see Angelopoulos et al. , 2008, and papers in this issue].48Figure 1 shows the pseudo-AE index, calculated fr om THEMIS ground based magne-49tometer data [Russell et al., 2 008] and the potential difference a cross the polar cap (∇Φ),50calculated from the SuperDAR N data, for March 23, 2007 . A visual comparison showed51a good agreement of the THEMIS pseudo-AE (further AE) and provisional AE from the52Kyoto monitor. Five THEMIS probes were at the same orbit with apogee at 14.7 RE.53During the inbound motion, t hey crossed the inner edge of the PS (the 1-5 keV electron54flux cut off) at LT≈23:10 with UT lags of 3 hours (between THC and THD), 10 min55(between THD and THB, and THB and THA), and 2.5 hours (between THA and THE).56TC-1 was in the dusk-side PS, crossing its inner edge simultaneously with the THDBA-57D R A F T April 7, 2008, 11:25am D R A F TX - 6 RUNOV ET AL.: INNER BOUNDARY OF THE PLASMA SHEETcluster a t LT≈20:00 (Figure 1, bottom panel). THC crossed the PS inner edge again at58LT≈16:30 during the outbound motion. All SC probed the PS inner edge during enhanced59activity: 100<AE<700 nT, and ∇Φ scattering between 45 and 70 mV (Figure 1, upper60panels).61Figure 2 presents ER spectrograms of 1 - 1000 keV ions and 0.1 - 25 keV electrons,62obtained by the THEMIS ESA [McFadden et al., 2008] and SST [La rs on et al. , 2008]63instruments during inbound