1 Reconstruction of a Flux Transfer Event Based on Observations From Five THEMIS Satellites A. T. Y. Lui JHU/APL, Laurel, MD 20723-6099, USA D. G. Sibeck NASA/GSFC, Greenbelt, MD, USA T. Phan Space Sciences Laboratory, University of California, Berkeley, CA, USA 10 V. Angelopoulos IGPP/UCLA, Los Angeles, CA, USA J. P. McFadden Space Sciences Laboratory, University of California, Berkeley, CA, USA K.-H. Glassmeier TUBS, Braunschweig, Germany 2 Abstract. We investigate a flux transfer event (FTE) observed by THEMIS near the duskside magnetopause using the reconstruction technique based on solving the Grad-Shafranov equation as developed 20 previously by Sonnerup et al. [2006]. THEMIS D detected the FTE with the largest core magnetic field. THEMIS B and C observed deep troughs in the magnetic field associated with the FTE. THEMIS A and E sensed only slightly altered magnetic field from their surroundings. Two- dimensional reconstruction maps of magnetic field and plasma pressure are generated by combining observations from all five THEMIS satellites. These reconstructed maps show distinct differences between a magnetic island and an FTE in terms of vector potential and the derived plasma parameters. Furthermore, the resulting maps show also asymmetry in these parameters between the magnetosheath and magnetospheric sides of the 30 FTE. This asymmetry contributes partially to the different characteristics of the FTE seen by the five THEMIS satellites in addition to the different impact parameters of these satellites with respect to the FTE center. 3 1. Introduction The most recent magnetospheric mission from NASA is THEMIS, an acronym for Time History of Events and Macroscopic Interactions during Substorms [Angelopoulos et al., 2008; Sibeck and Angelopoulos, 2008]. The primary objective of THEMIS is to resolve the controversy on the substorm initiation location in the magnetotail by placing 40 five satellites with identical instruments aligned along the tail axis to determine incontrovertibly the propagation direction of substorm disturbances in the magnetotail. Due to a launch delay of the mission, the planned separation of the five satellites along the tail axis was postponed for nearly one year. A decision was made to utilize this coast phase to have the five satellites placed in the ‘beads on a string’ configuration. Thus, a unique opportunity arises to probe magnetospheric structures in the near-Earth space with nearby multipoint measurements. In this coast phase, Sibeck et al. [2008] have identified a flux transfer event (FTE) near the dusk magnetopause on 2007 May 20. During this interval, one THEMIS satellite (THEMIS D) detected the FTE with a strong core magnetic field, while two another 50 THEMIS satellites (THEMIS B and C) detected a crater FTE in which the strong core magnetic field is bounded by deep troughs in magnetic field strength. FTEs with this signature are called crater FTEs. The two remaining THEMIS satellites (THEMIS A and E) detected only slight magnetic field strength increase. Assisted by the MHD simulation result of the event, Sibeck et al. [2008] have proposed that these different signatures of FTE arise from different impact parameters with respect to the center of the FTE by the 4 different satellites. THEMIS D had a very low impact parameter while THEMIS B and C had moderate impact parameters. THEMIS A and E had large impact parameters. Lui et al. [2008] have reconstructed the plasma parameters around the FTE event detected by THEMIS D using the Grad-Shafranov (GS) reconstruction technique 60 developed previously [Sonnerup and Guo, 1996; Hau and Sonnerup, 1999; Hu and Sonnerup, 2001, 2003; Teh and Hau, 2004, 2007; Hasegawa et al., 2004, 2005, 2006, 2007; Sonnerup et al., 2004, 2006]. They have benchmarked their reconstruction results with a theoretical model as well as with actual IRM magnetopause crossing data previously investigated by Hau and Sonnerup [1999]. From this procedure, Lui et al. [2008] have extended the observed magnetic field and plasma pressure of the FTE from THEMIS D to a two-dimensional plane. In addition, current densities associated with the FTE have been deduced in this two-dimensional plane. The result indicates that the axial current density had a peak value > 40 nA/m2 with significant current densities (up to ~25 nA/m2) on the plane perpendicular to its axis. 70 In this paper, we extend the previous GS reconstruction based on observations from a single THEMIS satellite to encompass observations from all five THEMIS satellites. The GS reconstruction technique has been extended to analyze observations from four Cluster measurements of magnetic islands near the magnetopause by Hasegawa et al. [2004, 2005]. Here, we extend the GS reconstruction technique even more, to encompass observations from five satellites. Naturally, the complexity in producing composite reconstruction maps increases with increasing number of satellites involved and thus is not a trivial extension. The resulting reconstruction maps provide insights on the observed FTE and the origin of the troughs in magnetic field seen in crater FTEs, which 5 have been reported and theorized previously by several researchers [Saunders et al., 80 1984; Rijnbeek et al., 1987; Farrugia et al., 1987; Ding et al., 1991; Sibeck et al., 2008]. 2. THEMIS observations 2.1 THEMIS D observations The flux transfer event (FTE) observations that provide the basis for the GS reconstruction reported here are exemplified by THEMIS D and C. Figure 1 shows the plasma and magnetic field measurements from THEMIS D that indicate the encounter of an FTE with an intense core magnetic field. The GSE coordinate system is used for these parameters. During this interval, the satellite crossed the magnetopause from the magnetosheath at the start of the interval to the magnetosphere at the end. The 90 magnetosheath region is indicated by the high plasma density, a relatively steady tailward plasma flow of ~200 km/s and fluctuating southward magnetic field. The magnetospheric region is indicated by the low plasma density, high and steady northward magnetic field, and a nearly stationary state of the plasma. At ~2202 UT within this interval, the plasma and magnetic field characteristics were quite distinct from both the magnetosheath and the magnetosphere regions. A prominent feature with a strong Bz magnetic field component accompanied by a much reduced tailward plasma flow was detected for ~1min. This structure is marked in Figure 1 by the region between the two vertical dashed lines. A dipolar signature can be seen in both the Bx and By components associated with this structure. These characteristics are commonly recognized as that of a FTE 100 [Russell and Elphic, 1978]. 6 2.2 THEMIS C observations THEMIS C was close to THEMIS D and detected the characteristics of a crater FTE as shown in Figure 2. At about the same time when THEMIS D detected the high core magnetic field of an FTE, THEMIS C detected a distinct feature with similar magnetic field and plasma flow characteristics. The structure is marked in Figure 2 by the region between the two vertical dashed lines. The only outstanding difference in characteristics from that seen by THEMIS D in Figure 1 is the presence of two deep minima in magnetic field strength bounding the core magnetic field. It can also be noted that the core 110 magnetic field is still high but is reduced substantially from that seen by THEMIS D. This field strength difference indicates that THEMIS C did not encounter the FTE as close to its center as THEMIS D. This inference is consistent with the presence of the deep minima [Sibeck et al., 2008]. 2.3 Observations from all THEMIS satellites Figure 3 provides a brief overview of observations from all five THEMIS satellites during this interval. It may be noted that observations from THEMIS B and C are very similar at ~2202 UT except that the magnetic field minima seen at THEMIS B were very pronounced without a clear indication of a peak in the magnetic field strength within it. 120 Although THEMIS A and E were both immersed well within the magnetosheath, there was an enhancement of the magnetic field at ~2202 UT for both satellites due to the passing of the FTE structure near the magnetopause. The locations of the five satellites in the GSE coordinate system are also given in Figure 3. 7 3. GS reconstruction of THEMIS observations 3.1 Underlying principle for reconstruction As discussed in Sonnerup et al. [2006] and Hasegawa et al. [2004, 2005], reconstruction of plasma configuration from observations by a single satellite and multiple satellites is based on solving the Grad-Shafranov (GS) equation assuming the 130 structure has a two-dimensional (2D) MHD equilibrium. The GS equation is # "2 "2 & dP A % ( A t ( ), % 2 + 2 ( = )µ0 $ "x "y ' dA 2 where the transverse pressure is given by Pt = p + Bz /2µ0. The magnetic field vector B is ! related to the partial vector potential A(x,y) and the axial magnetic field Bz by ! B = "A(x, y)# zˆ + Bz(A)zˆ . The third dimension is considered as the invariant axis, representing the direction along which the structure changes much more gradually in ! comparison to the variation on the plane perpendicular to it. The approach in solving the GS equation is treating it as a spatial initial value problem. The transverse pressure and the axial magnetic field component Bz are modeled by a combination of polynomial and exponential functions of the partial vector potential A(x,y). 140 3.2 Reconstruction procedure We have adopted the procedure documented in Hau and Sonnerup [1999] in the previous work to reconstruct two-dimensional maps of plasma parameters based on observations from a single satellite [Lui et al., 2008]. Our previous analysis indicates that a well-defined coordinate system can be obtained through minimum variance analysis and deHoffman Teller transformation.