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Swe, a Comprehensive Plasma Instrument for the Wind Spacecraft SWE, A COMPREHENSIVE PLASMA INSTRUMENT FOR THE WIND SPACECRAFT K. W. OGILVIE, D. J. CHORNAY, R. J. FRITZENREITER, F. HUNSAKER, J. KELLER, J. LOBELL, G. MILLER, J. D. SCUDDER and E. C. SITTLER, JR. Interplanetary Physics Branch, Goddard Space Flight Center, Greenbelt, MA 20771, U.S.A. R. B. TORBERT, D. BODET and G. NEEDELL Physics Department and Space Science Center, Institute for Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03284, U.S.A. A. J. LAZARUS, J. T. STEINBERG and J. H. TAPPAN Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. and A. MAVRETIC and E. GERGIN Microelectronics Research Laboratory, Boston University, Boston, MA 02215, U.S.A. (Received 27 May, 1993) Abstract. The Solar Wind Experiment (SWE) on the WIND spacecraft is a comprehensive, integrated set of sensors which is designed to investigate outstanding problems in solar wind physics. It consists of two Faraday cup (FC) sensors; a vector electron and ion spectrometer (VEIS); a strahl sensor, which is especially configured to study the electron 'strahl' close to the magnetic field direction; and an on-board calibration system. The energy/charge range of the Faraday cups is 150 V to 8 kV, and that of the VEIS is 7 V to 24.8 kV. The time resolution depends on the operational mode used, but can be of the order of a few seconds for 3-D measurements. 'Key parameters' which broadly characterize the solar wind positive ion velocity distribution function will be made available rapidly from the GGS Central Data Handling Facility. 1. Introduction The solar wind experiment (SWE) for the WIND spacecraft is a comprehensive, integrated set of instruments which will attack many outstanding problems in the magnetosheath, the foreshock, and the interplanetary medium. It also has the capability of making measurements in other regions which might become available to it. The 'key parameters' (velocity, density, and temperature of the solar wind ions) will be extracted from detailed three-dimensional measurements made by the Faraday cup (FC subsystem; they will be made available rapidly through the GGS Central Data Handling Facility. This part of the instrument derives from the Voyager plasma instrument (Bridge et al., 1977) and the Faraday cup instruments on IMP 7 and 8 (Bellomo and Mavretic, 1978). The FC subsystem is particu- larly suited for measurement of flowing, supersonic plasma such as that found in the interplanetary medium. Table I lists the key parameters and the expected measurement precisions. Space Science Reviews 71: 55-77, 1995. @ 1995 Kluwer Academic Publishers. Printed in Belgium. 56 K.W. OGILVIEET AL. TABLE I Key parameters from the SWE instrument Parameter Range Precision Proton velocity 200-1250 km s -1 4-3% (3 components) Proton number density 0.1-200/cc +10% Thermal speed 0-200 km s -I -t-10% Alpha/proton number 0-100% 4-10% density ratio x 1013 The foreshock electrons and ions reflected from the bow shock will be studied using a vector electron and ion spectrometer (VEIS) derived from ISEE-1 instru- mentation (Ogilvie et al., 1978) but with increased sensitivity which is sufficient to make more detailed measurements. It will obtain highly-time-resolved, three- dimensional snapshots of the distribution functions of ions and electrons having energies/charge from 7 V to 24.8 kV for flowing plasmas with Mach numbers of unity and below. A property of the solar wind electron velocity distribution which has not received sufficient study at 1 AU is the field-aligned distortion of the electron distribution function. That distortion, known as the' strahl', has been interpreted to be a remnant of the electron distribution in the solar corona (Scudder and Olbert, 1979a, b) and is thus of considerable theoretical interest. The SWE has a specialized subsystem, the strahl detector, to study this phenomenon, by making detailed measurements of the electron velocity distribution near the direction of the magnetic field. Such a comprehensive instrument, with many possible modes of operation, must be controlled by a correspondingly flexible data processing and control unit (DPU). The DPU, has ample computational capacity for the control of many possible modes, for data storage and formatting for presentation to the telemetry system, for command decoding, and for some data analysis. (For example, it is possible to calculate approximate key parameters on-board the spacecraft.) This paper consists of a compact discussion of the scientific aims of the SWE, followed by a brief description of each of the subsystems; our intent is to give sufficient detail to allow preliminary assessment of the data obtained by them. 2. Scientific Objectives The scientific aims of the investigation include those directly related to the GGS program of investigating the transfer of energy and momentum from the Sun to the earth and those which concern the properties of interplanetary space and are SWE, A COMPREHENSIVE PLASMA INSTRUMENT FOR THE WIND SPACECRAFT 57 independent of the presence of Eanh. We outline briefly the major objectives of our experiment in the following paragraphs. 2.1. STUDIES OF THE ENERGY AND MOMENTUM INPUT TO THE MAGNETOSPHERE Of particular interest to the GGS program are measurements of the solar wind velocity (three components), density, and average thermal speed for which Faraday cups are especially suitable. The incoming energy and momentum available for transfer to the magnetosphere can then be computed (assuming that the single- point measurement at WIND is representative of conditions over the cross section of the magnetosphere), and changes in these quantities can be compared with time-correlated changes in the magnetosphere. 2.2. MAGNETOSPHERIC EFFECTS OF UPSTREAM WAVES AND PRESSURE CHANGES There is currently abundant evidence that many magnetospheric phenomena are caused by a combination of changes in the bulk velocity and number density of the solar wind together with changes in the direction of the incident magnetic field. Those variations in the incident wind are often difficult to identify clearly as causes of specific magnetospheric events without observations upstream in the solar wind. The continuous coverage, good time resolution, and good determination of the bulk velocity, provided by the SWE are expected to provide inputs needed by magnetospheric modelers. The provision of these parameters are the main GGS- related scientific aim. 2.3. MEASUREMENTS IN THE REGION BETWEEN THE L1 LAGRANGIAN POINT AND THE BOW SHOCK Even though WIND spends a good portion of its time far from Earth-related disturbances (such as ions reflected from the bow shock), it will still not be possible to correlate changes in the solar wind seen near L1 with magnetospheric events unless the region between L1 and the bow shock is thoroughly explored. That exploration should include identification of wave types and conditions for their generation so that the changes occurring as the solar wind approaches Earth can be incorporated in models which can then give a clear picture of the environment just upstream of the magnetosphere. Identification of the incoming waves requires the close, cooperative analysis of data from different WIND experiments as well as those from other spacecraft envisioned in the structure of GGS and made possible by ready access to data in the ground data system. The region upstream of the bow shock, called the 'foreshock', is described in a compilation 'Upstream Waves and Particles' in J. Geophys. Res. 86, 4319, 1981. 58 K.W. OGILVIE ET AlL. 2.4. Bow SHOCK AND MAGNETOSHEATH STUDIES Studies of the bow shock and the magnetosheath, especially near the stagnation point, can be performed during the early part of the mission when the spacecraft makes multiple traversals of the bow shock region on both sides of the noon meridian. The (VEIS) and the Faraday cups (FC) together can cover a Mach number range from zero to at least 20; and the SWE has a mode in which traversals can be captured rapidly and played back more slowly. Of particular interest are studies of multiple crossings of the bow shock and evidence for reconnection events. 2.5. MEASUREMENT OF VELOCITY DISTRIBUTION FUNCTIONS OF PROTONS, HELIUM IONS~ AND ELECTRONS The SWE complement of sensors makes possible measurement of the three- dimensional velocity distribution functions of the protons and alphas. Two FC sensors provide cuts through the wind' s velocity distribution function integrated in a plane perpendicular to the cup normal (see, for example, Vasyliunas, 1971). The two Faraday cup normals are placed 15 ~ above and below the equatorial plane of the spacecraft thus providing three-dimensional scans of the ion distribution function. The VEIS instrument measures three-dimensional velocity distribution func- tions for flows with Mach numbers less than unity. The electrons in the solar wind, magnetosheath and magnetosphere, and ions reflected diffusely from the bow shock satisfy this condition. The instrument, derived from the electron spectrometer on the ISEE-1 spacecraft, can supply accurate meas- urements of the electron heat flux, including its direction and the flow direction of the electrons (see Figure 1). The SWE strahl sensor is able to make detailed measurements of the electron velocity distribution along the magnetic field direction where the distribution is distorted, reflecting the non-local nature of those electrons (see further discussion in Section 3.5). 2.6. HIGHLY-TIME-RESOLVED MEASUREMENTS OF THE SOLAR WIND As discussed later, measurement of the entire ion velocity distributions can be made as the spacecraft rotation carries a Faraday cup with its wide acceptance angle of q-60 ~ through the solar wind direction in approximately 1 s. This mode of operation promises to extend measurement of the solar wind parameters into a time regime that has not yet been thoroughly explored. 2.7. STUDIES OF INTERPLANETARY SHOCKS AND INTERACTION REGIONS In addition to studying the nature of the bow shock, the 'burst' or 'event' mode of the SWE instrument can be used to observe phenomena in the vicinity of other boundaries in the plasma.
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