THEMIS ESA First Science Results and Performance Issues J. P. McFadden, C. W. Carlson, D. Larson, J. Bonnell, F. Mozer, V. Angelopoulos, K.-H. Glassmeier, U. Auster University of California Space Sciences Laboratory 7 Gauss Way Berkeley, CA 94720 [email protected] 510-642-9918 office 510-643-8302 faxTHEMIS ESA First Science Results and Performance IssuesAbstract Early observations by the THEMIS ESA plasma instrument have revealed new details of the dayside magnetosphere. As an introduction to THEMIS plasma data, this paper present observations of plasmaspheric plumes, ion conic outflows, field line resonances, structure at the low latitude boundary layer, flux transfer events at the magnetopause, and wave and particle interactions at the bow shock. These observations demonstrate the capabilities of the plasma sensors and the synergy of its measurements with the other THEMIS experiments. In addition, the paper includes discussions of various performance issues with the ESA instrument such as sources of sensor background, measurement limitations, and data formatting problems. These initial results demonstrate successful achievement of all measurement objectives for the plasma instrument. Keywords THEMIS, Magnetosphere, Magnetopause, Bow Shock, Instrument Performance Classification System 94.80.+g 06.20.fb 94.30.C- 94.05.-a 07.87.+v1.0 Introduction The THEMIS mission provides the first multi-satellite measurements of the dayside magnetosphere, magnetopause and bow shock with a string of pearls orbit near the ecliptic plane (Angelopoulos et al., 2008). Each THEMIS spacecraft includes a fluxgate magnetometer (Auster et al., 2008), a search coil magnetometer (Roux et al., 2008), a 3-axis electric field instrument (Bonnell et al., 2008), solid state telescopes (SST) for energetic (>30 keV) ions and electrons (Larson et al., 2008), and electrostatic analyzers (ESAs) for electron and ion plasma (<30 keV) measurements (McFadden et al., 2008a). These instruments not only provide the information needed to perform substorm timing analysis during the prime mission, but also provide a core set of measurements needed to resolve most magnetospheric dynamics. In this paper we will highlight the measurement capabilities of the ESA plasma sensors by presenting “first results” from several regions in and around the magnetosphere. Although plasma measurements will be the focus of these first results, observations from the other sensors are included to illustrate the synergy of these measurements and to demonstrate the ability of THEMIS satellites to resolve the basic plasma features of these regions. Throughout the paper we refer to the individual satellites by their abbreviated call letters – THA, THB, THC, THD and THE. The THEMIS plasma instruments measure the 3-D plasma distribution function with ~3 s resolution. Although the highest-time, highest-phase-space resolution measurements are only available during bursts which are generally limited to ~15 minutes per orbit, coarser 3-D distributions at spin resolution are available for ~12 hours each orbit. These data have adequate angular resolution to allow accurate ground computation of moments and identification of beams. Even during periods where data collection is limited, the ESA data products include on-board calculated moments and omni-directional energy spectra at spin resolution. These on-board moments include corrections for spacecraft charging providing accurate electron moment computations that eliminate photoelectrons. Spin-resolution energy spectra provide the additional information needed to interpret variations in the on-board moments and to correctly identify the dynamics associated with boundaries or changes in the multi-component plasma. As demonstrated in the accompanying paper (McFadden et al., 2008a), the close proximity of the five THEMIS spacecraft during the early mission allowed very accurate in-flight calibration of the ESA sensors. The relative sensitivities of the ten sensors are believed to be determined to better than 5%, and the absolute sensitivity corrected to ~10% through cross calibration with Wind-SWE. The primary uncertainties in this calibration effort resulted from estimation errors of the proton to alpha ratio of the solar wind, from uncertainties in calculating the spacecraft-to-plasma potential from the measured spacecraft-to-langmuir-probe potential, and by relying on the literature to correct for energy dependent efficiencies of the microchannel plate detectors. However, pressure balance checks across the dayside magnetopause (Figure 16, McFadden et al., 2008a), performed independent of the calibration effort, provide additional confidence in our techniques. These accurate calibrations allow the combined electron and ion data tobe used to deduce additional features about the plasma including mass composition and the presence of missed cold plasma. During the first 7 months of the THEMIS mission, the five spacecraft sampled the dusk, sub-solar and dawn regions of the magnetosphere. Section 2 presents first results from the encounters with these regions illustrating the abilities of the instruments to resolve small scale features and separate space and time. Although the THEMIS ESAs provide a data set of well calibrated observations, there are still several instrumental limitations to the data. During the course of the presentation, we will point out measurement limitations and uncertainties in the observations that can affect the accuracy of computed products such as moments. Some of these performance issues involve missing information, such as composition, while others are associated with the instrument’s dynamic range. THEMIS has an open data policy that strives for an immediate data release to the community. While data quality flags will be inserted into high level processed data, much of the data analysis effort will utilized unprocessed data. Therefore scientists need a reference where performance issues are identified, such as non-geophysical background counts, or where the impact of missing information, such as composition, is discussed. In section 3 we provide a summary of all known performance issues with the ESA sensor including sources of background, non-ideal response of the instrument, limitations due to missing information, and telemetry formatting problems. Understanding and correcting for these performance issues will allow full use of the