Article File #1page 2page 3page 4page 5page 6page 7page 8page 9page 10page 11page 12Figures_1-4page 2page 3page 41THEMIS observations of penetration of the plasma sheet into the ring current region 1 during a magnetic storm 2 3 4 Chih-Ping Wang and Larry R. Lyons 5 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA 6 Vassilis Angelopoulos 7 Department of Earth and Space Sciences, University of California, Los Angeles, California, USA 8 Davin Larson, J. P. McFadden, and Sabine Frey 9 Space Sciences Laboratory, University of California, Berkeley, USA 10 Hans-Ulrich Auster 11 Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Germany 12 Werner Magnes 13 Space Research Institute, Austrian Academy of Sciences, Graz, Austria 14 15 162Abstract. Observations from the THEMIS spacecraft during a weak magnetic storm clearly 1 show that the inner edge of the ion plasma sheet near dusk moved from r ~6 RE during the pre-2 storm quiet time to r ~3.5 RE during the main phase, and then moved outward during the 3 recovery phase. The plasma sheet particles from the tail reached the inner magnetosphere during 4 the main phase through open drift paths, which extended earthward with increasing convection, 5 and were energized to ring current energies, resulting in an increase in ring current population. 6 During the recovery phase, the plasma sheet region retreated to outside the inner magnetosphere 7 as the region of open drift paths moved outward with decreasing convection, leaving the ring 8 current particles within the expended closed drift path region. With no new particles from the 9 plasma sheet, ring current population gradually decreased due to losses as the recovery phase 10 proceeded. 11 1. Introduction 12 Plasma sheet particles, which originally come from the solar wind and ionosphere and are 13 within the region of open electric and magnetic drift paths, have been assumed to be the major 14 particle source for the ring current population (10 –200 keV ions and electrons) in particle 15 simulations for the ring current [e.g., Chen et al., 2006]. According to the models, the inner edge 16 of the plasma sheet (the separatrix between the regions of open and closed particle drift paths) 17 should move earthward when the convection electric field is enhanced, such as during a storm 18 main phase, thus allowing the particles from the tail plasma sheet to have access to the inner 19 magnetosphere and to be adiabatically energized to the ring current energy range. Once inside 20 the inner magnetosphere, higher energy particles can be further energized by radial and energy 21 diffusion, but it is the earthward movement and adiabatic energization of plasma sheet particles 22 that is critical to ring current enhancement within the models. The CRRES Observations in the 233region of r < ~5.3 RE [Korth et al., 2000] have supported the above hypothesis, but didn’t 1 directly identify the ring current population as plasma sheet particles. 2 The inbound passes of the THEMIS spacecraft from r ~15 to ~2 RE at the same near-dusk local 3 times during different phases of a weak storm in May, 2007 (minimum Dst = –60 nT on May 23, 4 which is the strongest storm in 2007 since the THEMIS spacecraft began operations in March 5 2007) provide unprecedented end to end measurements of the equatorial plasma sheet from its 6 outward boundary at the magnetopause, to its earthward boundary, and into the ring current 7 region, which allows us to obtain an unambiguous determination of the radial extend of the 8 plasma sheet and its variation throughout the storm. The THEMIS results clearly show that the 9 plasma sheet penetrated earthward into the inner magnetosphere during the storm main phase 10 and plasma sheet particles were energized to within the ring current energy range, resulting in an 11 increase in the number of ring current particles. The observations are consistent with particle 12 distributions resulting from large-scale electric and magnetic drift. This clearly indicates that the 13 majority of the ring current particles were originally plasma sheet particles that moved to the 14 inner magnetosphere along the open drift paths, as assumed in the models. 15 2. THEMIS observations during the storm 16 The Dst and the interplanetary conditions from Wind for the storm are shown in Figure 1a. The 17 THEMIS mission consists of five spacecraft orbiting near the equatorial plane [Angelopoulos, 18 2008]. The ions and electrons are measured by an electrostatic analyzer (ESA, 0.006 – 20 keV/q 19 for ions and 0.007 – 26 keV for electrons [McFadden et al., 2008]) and a solid state telescope 20 (SST, 28 keV – 6 MeV for both ions and electrons [Larson et al., 2008]). The magnetic field (3 21 seconds average of spin fitted data) is measured by the FGM instrument [Auster et al., 2008]. 22 Throughout the storm, the multiple THEMIS spacecraft were lined up and passed through nearly 234the same locations about once per day. The trajectories of the THEMIS spacecraft (moving 1 toward smaller radial distance) during five different phases (pre-storm quiet time, sudden 2 commencement, main phase, early recovery, and late recovery) are shown in Figure 1b. For the 3 five phases, we used data from THEMIS B, E, D, D, B, respectively for the best data coverage 4 and quality. The magnetic field strengths as a function of radial distance r are shown in Figure 5 1c, and it can be seen that the magnetic fields in the inner magnetosphere (r < 6 RE) became 6 weaker during the main phase due to the enhanced ring current. (r is computed from the 7 projection of the spacecraft positions unto the X-Y GSM plane. The ZGSM of the spacecraft is < 8 ~2 RE and is smaller closer to the Earth.) 9 We define the ring current population as ions and electrons of energy from 30 to 200 keV in 10 the region between r = 2 and 7 RE. Figure 2 shows the spectrums of ominidirectional energy 11 fluxes as a function of r during the five different phases of the storm. During the pre-storm quiet 12 time, it can be seen from Figure 2a that the energy fluxes changed sharply at r ~13.7 RE in both 13 ions and electrons as the spacecraft moved across the magnetopause from the colder 14 magnetosheath to the hotter magnetosphere. Inside the magnetosphere, the spacecraft 15 continuously observed similar magnitudes of ion energy fluxes peaking at ~20 keV as it moved 16