Revised Version: 2004-07-27, Section 8 on PI contact information ------------------------------------------------------------------ USERS GUIDE FOR ULYSSES SWOOPS DATA FROM THE JUPITER ENCOUNTER TABLE OF CONTENTS 1. OVERVIEW OF THE SWOOPS EXPERIMENT 2. MEASUREMENTS AT JUPITER 3. THE SWOOPS ELECTRON EXPERIMENT 4. DATA REDUCTION ALGORITHMS 5. BOUNDARIES AND TRANSITIONS OBSERVED DURING THE ENCOUNTER 6. DESCRIPTION AND FORMAT OF DATA SUBMITTED TO THE NSSDC 7. DATA CAVEATS 8. FURTHER INFORMATION 1. OVERVIEW OF THE SWOOPS EXPERIMENT The SWOOPS (Solar Wind Observations Over the Poles of the Sun) experiment has two electrostatic analyzers, one for positive ions and one for electrons. The instrument is fully described in: The Ulysses Solar Wind Plasma Experiment, S. J. Bame, D. J. McComas, B. L. Barraclough, J. L. Phillips, K. J. Sofaly, J. C. Chavez, B. E. Goldstein, and R. K. Sakurai, Astronomy and Astrophysics Supplement Series, Ulysses Instruments Special Issue, Vol. 92, No. 2, p. 237-265, 1992. The electron and ion analyzers are separate instruments that operate asynchronously. 2. MEASUREMENTS AT JUPITER The following publications contain descriptions of SWOOPS operations and instrumental limitations at Jupiter: Phillips, J.L., S.J. Bame, M.F. Thomsen, B.E. Goldstein, and E.J. Smith, Ulysses Plasma Observations in the Jovian Magnetosheath, Journal of Geophysical Research, p. 21189, 1993. Phillips, J.L., S.J. Bame, B.L. Barraclough, D.J. McComas, R.J. Forsyth, P. Canu, and P.J. Kellogg, Ulysses Plasma Electron Observations in the Jovian Magnetosphere, Planetary and Space Science, p. 877, 1993. The electron instrument functioned well throughout the encounter, subject of course to its inherent energy limitations and to the energetic particle background present in the magnetosphere. The ion experiment, which was optimized for solar wind measurements and thus has a limited field of view, produced useful measurements in the outbound magnetosheath only. A summary of the ion measurements follows: INBOUND MAGNETOSHEATH. The look direction of the experiment was ground- commanded, based on expectations of sheath flow direction. During the brief inbound sheath encounter, the flow was anomalous (sunward at times). Thus the instrument "missed the beam" and returned no useful measurements. OUTBOUND MAGNETOSHEATH. Here the flow was more or less as expected. The ion experiment captured most of the sheath flow beam during most outbound sheath intervals. The measurements are shown in the sheath paper cited above. In general, flow velocities agreed with the electron velocities. Disagreements occurred when the ion sensor missed the heart of the beam. Densities and temperatures were generally substantially underestimated by the ion experiment due to its limited angular coverage. MAGNETOSPHERE. Due to the limited energy range and look direction of the instrument, as well as the energetic particle background, the ion instrument did not return useful data in the magnetosphere. As a result of these problems, the SWOOPS ion observations inside the Jovian bow shock are not included in the archived Jupiter data. Data provided start at the beginning of day 25 (January 25, 1992) and run through the end of day 48 (February 17) and include all bow shock and magnetopause crossings. Both experiments were shut down for a 25-hour interval surrounding closest approach due to elevated penetrating background. The interval specified by PDS for this encounter includes substantial intervals of solar wind. In general, the SWOOPS ion experiment provides superior characterization of solar wind density and speed in the solar wind, while the electron experiment provided the only easily usable data inside the Jovian bow shocks. In order to provide the best observations for each phase of the encounter, the following schemes were adopted: 0000 UT on Day 25 until 1200 UT on Day 33: Solar wind scheme. Electron temperature, charge-weighted ion (proton + alpha) density, and proton velocity. Ion data were averaged to coincide with the electron spectral times. 1200 UT of Day 33 until 1200 UT on Day 47: Jupiter scheme. Electron temperature, density, and velocity 1200 UT on Day 47 until 2400 UT on Day 48: Solar wind scheme. The minor plasma discontinuities occurring at 1200 UT on days 33 and 47 are artifacts of the switchover between Jupiter and solar wind schemes. While the solar wind ion products included in the archive are created through routine data reduction, the electron parameters required unique Jupiter- specific processing. A description of the experiment and processing follows. 3. INTRODUCTION TO THE SWOOPS ELECTRON EXPERIMENT The SWOOPS electron spectrometer is a 120-degree spherical section electrostatic analyzer which measures the 3-d velocity space distributions of solar wind electrons. In the mode used at Jupiter, the instrumental energy range was 1.6 to 862 eV in the spacecraft frame. Since the spacecraft charged to +2 to +44 volts during the encounter, 2 to 44 eV are subtracted from the measured energies (electrons measured at energies below the spacecraft potential are electrostatically trapped photoelectrons). Each spectrum takes 2 minutes to accumulate, but telemetry takes longer. During the Jupiter encounter, spectra were returned every 5.7 minutes. The analyzer uses 7 channel electron multipliers (CEMs) to count electrons discretely over 95% of the unit sphere in look direction. For telemetry conservation, 2 out of every 3 spectra are "two-dimensionalized" onboard the spacecraft, that is the count rates are averaged over all 7 CEMs. These 2-d spectra thus return electron counts as a function of energy and spacecraft spin angle. The full 3-d spectra return counts as a function of energy, spacecraft spin angle, and polar angle (measured from the spacecraft spin axis, which points at Earth). The 3-d spectra incorporate 32 spin-angle steps, for a total spectral content of 20 energies x 32 azimuths x 7 polar angles, while the 2-d spectra include 20 energies x 64 azimuths x 1 angle. 4. ELECTRON DATA REDUCTION ALGORITHMS The first step in data reduction is determination of the spacecraft potential. This is done by identifying inflections in the angle-averaged energy spectra. While this is automated in the solar wind, it was done by eye, spectrum-by- spectrum, for the Jupiter encounter. Potential averages +6 V in the solar wind and sheath, and substantially higher in the magnetosphere. The count rate arrays are corrected for spacecraft potential and converted to phase- space density arrays. Plasma moments are then calculated by numerical integration of the velocity- weighted ion distributions. A total integration is performed from the spacecraft potential (corresponding to zero energy solar wind electrons) to the instrumental energy limit. Since the first few eV above the spacecraft potential are contaminated with photoelectrons on non-radial trajectories, it is necessary to use a biMaxwellian fit to the lowest energy points to fill in this part of the distribution. The integrations produce density, temperature components, velocity, and heat flux. At this time, densities, scalar temperatures, and 3-d velocity vectors (for 3-d measurements only) are being provided for archiving. 5. BOUNDARIES AND TRANSITIONS OBSERVED DURING THE ENCOUNTER The following boundaries and regions were identified by the SWOOPS experiment team (see the papers cited above for details): Day Time Feature ---------------------------------------------------------------- 33 1733 Cross bow shock (BS) into magnetosheath 33 2222 Cross magnetopause (MP) into boundary layer (BL) 33 2308 Enter magnetosphere 34 1655 Enter BL 34 1720 Cross MP into sheath 34 1945 Cross MP into BL 35 0025 Cross MP into sheath 35 0100 Cross MP into BL 35 0125 Cross MP into sheath 35 0250 Cross MP into BL 35 0400 Enter magnetosphere 38 1132 Enter open field region 38 1335 Enter magnetosphere 38 2137 Enter open field region 38 2310 Enter magnetosphere 39 0054 Last usable data before shutdown 40 0152 First usable data after shutdown 40 1126 Enter open field region 40 1232 Enter magnetosphere 40 2125 Enter open field region 40 2241 Enter magnetosphere 43 0024 Enter BL 43 0100 Enter magnetosphere 43 1058 Enter BL 43 1226 Enter magnetosphere 43 1337 Enter BL 43 1357 Cross MP into sheath 43 1700 Cross MP into BL 43 1740 Enter magnetosphere 43 1820 Enter BL 43 1910 Cross MP into sheath 45 0037 Cross BS into solar wind 45 0428 Cross BS into sheath 45 0933 Cross MP into BL 45 1030 Enter magnetosphere 45 1400 Enter BL 45 1600 Enter magnetosphere 45 1815 Enter BL 45 1825 Enter magnetosphere 45 2045 Enter BL 45 2140 Cross MP into sheath 47 0753 Cross BS into solar wind 6. DESCRIPTION AND FORMAT OF DATA SUBMITTED TO THE NSSDC The data provided to the NSSDC are the total charge density (electron or charge-weighted ion), electron temperature, and plasma (electron or proton) velocity. Electron data were integrated over the full instrumental energy range but not extrapolated to higher energies. Spacecraft position is also provided. The times specified in the file are the centers of each 2-minute spectrum. The data file was created with Fortran on a Vax running VMS. It can be opened and read as follows: open (3, file='JUPITER_ELECTRONS.DAT', status='old',recl=151) c iyr - year c idoy - day of year (Jan 1 = 001) c ihr - hour, UT c imin - minute, UT c isec - second, UT c idim - dimension of electron spectrum (2 or 3) c rj - Jupiter-spacecraft distance, Rj c xlat - Jovigraphic latitude of spacecraft, degrees c xmlat - magnetic latitude of spacecraft, degrees c lt - local time of spacecraft, hhmm c density - charge density per cubic cm c temp - electron temperature, Kelvins c vx,vy,vz - plasma bulk velocity, in km/s in XYZ system c vr,vtheta,vphi- plasma bulk velocity, in km/s in spherical system read(31,88) > iyr,idoy,ihr,imin,isec,idim,rj,xlat,xmlat,lt, > density,temp,vx,vy,vz,vr,vtheta,vphi 88 format(1x,i2,1x,i3,3(1x,i2),1x,i1,f8.3,2f8.2,i5.4,8e13.5) Velocity components are flagged (1.e32) for 2-d spectra when Jupiter scheme is in use. The following coordinate systems are used: 1. XYZ is a Jupiter-centered cartesian system with solar longitude fixed. Z is northward along the planetary rotation axis, X is perpendicular to Z in the plane containing Z and the Sun-Jupiter line, positive antisunward, and Y completes the right-handed set, positive dawnward. 2. Spherical is a standard spherical system based on the Jupiter-centered spacecraft position, with R positive outward, Theta positive in a southward sense, and Phi positive in the sense of planetary rotation. 7. DATA CAVEATS In the magnetosphere, a very simple scheme was used for background rejection. The lowest count rate for a given spectrum was subtracted from all pixels in that spectrum. Thus any changes in background during the course of a 2-minute spectra accumulation time are not accounted for. While the resulting plasma densities were corroborated by comparison with the plasma frequency-based densities from the URAP experiment (see magnetosphere paper cited above), the possibility exists that some roll-modulated background exists and distorts the plasma velocities. At the time of this submission, the velocities provided are the best values available. However, the SWOOPS team is actively pursuing a roll-modulated background rejection scheme, and the data will be updated as appropriate. The user should be cautious in interpreting the electron velocities. During the intervals indicated as open field regions in the listing above, there are significant uncertainties in all plasma parameters. This is due to uncertainties in the spacecraft potential. While it is clear that the densities are relatively high and the temperatures are low in these regions, small errors in spacecraft potential can create large errors in all derived plasma parameters. These values will be updated as the SWOOPS team research into the open field regions continues. The spacecraft ephemeris was provided by the Ulysses project at high resolution during the encounter proper. Far from the planet (i.e. the beginning and ending of the SWOOPS data file), the ephemeris resolution was coarser, and you will see large steps in spacecraft position in the SWOOPS data file. Higher-resolution position information may be available through the PDS. 8. FURTHER INFORMATION For information on this data set and on acquiring other types of data not provided to the NASA Planetary Data System or NSSDC, contact the SWOOPS Principal Investigator, Dr. David J. McComas, at Southwest Research Institute, dmccomas@swri.edu, 210-522-5983. For information on the reduction and analysis of data from the positive ion experiment, contact Dr. Bruce E. Goldstein at the Jet Propulsion Laboratory, bgoldstein@jplsp2.jpl.nasa.gov, 818-354-7366.