SWOOPS/Electron - User Notes

SWOOPS Contact:

Dr. Bruce E. Goldstein
Address: MS-169-506
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
USA

Phone: (1) 818 354-7366
Telefax: (1) 818 354-8895

E-mail: bgoldstein@jplsp2.jpl.nasa.gov

     NSSDC USER'S GUIDE FOR DATA FROM THE ULYSSES SWOOPS PLASMA EXPERIMENT: THE ELECTRON EXPERIMENT TABLE OF CONTENTS 1. OVERVIEW OF THE SWOOPS EXPERIMENT 2. INTRODUCTION TO THE SWOOPS ELECTRON EXPERIMENT 3A. INSTRUMENT OPERATION-SUMMARY 3B. INSTRUMENT OPERATION-DETAILED DESCRIPTION 4. DATA REDUCTION ALGORITHMS 5. DESCRIPTION AND FORMAT OF DATA SUBMITTED TO THE NSSDC 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. For this reason, the data from the two sensors are submitted separately to the NSSDC. This document describes the electron analyzer and the data submitted to the NSSDC for that analyzer; the ion experiment is described in a comparable document that accompanies the ion data. 2. 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 its normal solar wind mode, the instrumental energy range is 1.6 to 862 eV in the spacecraft frame. Since the spacecraft generally charges to +2 to +15 volts, 2 to 15 eV is subtracted from the measured energies (electrons measured at energies below the spacecraft potential are electro- statically trapped photoelectrons). The beginning of scientifically useful SWOOPS data is at the beginning of Day 322 of 1990. The electron instrument is subject to a "sleep mode" triggered by changes in the spacecraft configuration. The first time this mode occurred it caused a long gap in the electron (but not the ion) data: from 0621 UT on day 346 of 1990 until 2052 UT on day 362. Subsequently, procedures were developed to recognize, correct, and prevent this sleep mode, thus minimizing its impact. However, there are occasional gaps in the electron data when the SWOOPS ion sensor, and other Ulysses experiments, were returning data. 3A. INSTRUMENT OPERATION-SUMMARY Each spectrum takes 2 minutes to accumulate, but telemetry takes longer. When the spacecraft is being actively tracked and is returning data at its highest bit rate, spectra are returned every 2.3 minutes (low angular resolution mode) or every 5.7 minutes (high resolution mode). During playback of stored data, the spectral repetition rate is every 4.7 minutes (low angular resolution) or every 11.3 minutes (high resolution). These modes will be described in the next paragraph. For most of the mission, the instrument has been in high angular resolution mode, resulting in spectra every 5.7 minutes when the spacecraft is being tracked, or every 11.3 minutes for playback. The amount of spacecraft tracking depends on the mission phase. 3B. INSTRUMENT OPERATION-DETAILED DESCRIPTION The electron analyzer is provided with a 22-level high voltage supply to cover a range of ion energies from 0.8 to 862 eV. At any given time, either the top 20 or bottom 20 of these voltage levels are used. Except from a period from instrument turn-on in November 1990 through December 3, 1990, the instrument has been in high-energy mode throughout the mission, resulting in an energy range of 1.6 to 862 eV. Prior to that time, the instrument alternated between low-energy (0.8 to 454 eV) and high-energy (1.6 to 862 eV) spectra. 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). There are two angular resolution schemes which are ground commanded. In the high resolution scheme, which has been used for most of the mission, the 3-d spectra incorporate 32 spin-angle steps, for a total spectral content of 20 energies x 32 azimuths x 7 polar angles. The 2-d spectra incorporate 64 spin angle steps, for a total of 20 energies x 64 azimuths x 1 CEM-averaged polar angle (90 degrees from the spin axis). In the low resolution scheme, both 2-d and 3-d spectra incorporate only 16 spin angle steps, but provide a higher spectral repetition rate, as described in the previous section. Because spacecraft potential effects vary with polar angle, and the 2-d spectra sum over polar angle, it is not possible to correct fully for these effects using the 2-d spectra. The SWOOPS team has not found the 2-d spectra to be useful and they are not included in the NSSDC submission. During the highest spacecraft bit rate periods and highest instrument angular resolution, one 3-d spectrum is returned every 17.1 minutes. 4. DATA REDUCTION ALGORITHMS Calculation of the electron moments are strongly effected by the presence and variability of the assymetric spacecraft potential sheath.  Ulysses electron moments have been calculated to minimize these effects, however, some uncertainties remain in the inversion of these observations.  Our first step in the electron data reduction process is determination of the bulk, scalar spacecraft potential.  This is done by identifying inflections in the angle-averaged energy spectra.  The spacecraft potential averages +6V, with higher values for low-density plasma and lower (but still positive) values for high-density plasma. A second inflection is also identified in the spectra, corresponding to the break between thermal ("core") and suprathermal ("halo") populations. The count rate arrays are then corrected for spacecraft potential and converted to phase-space density arrays using the "plane-parallel correction" (e.g., Scime et al., JGR, p. 14769, 1994), a correction which attempts to unfold not only the energies but also the direction of motion of the electrons from thier observation point at the instrument apperture to well outside the spacecraft sheath. At high heliographic latitudes scattering of light into the sensor was intermittently observed, adversely effecting calculations of halo electron properties.  When this condition occurs, it is patched by interpolation from neighboring data measurements that are not contaminated by light. 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. Analogous core and halo integrations are performed for the parts of the distribution above and below the core-halo energy break point. 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 core distribution to fill in this part of the distribution; the total and core integration results are corrected based on this fit. The integrations produce density, temperature components, velocity, and heat flux. Uncertainties in the spacecraft potential and sheath configuration create errors in the density and temperature calculations. These problems are particularly severe when the solar wind is rarefied and cold, making it difficult to separate the photoelectron and thermal distributions. The halo density and core and halo temperatures are much less affected by the photoelectron effects than is the core density. The SWOOPS ion instrument provides much more accurate measurements of solar wind bulk density.  Therefore, assuming charge nutrality, we use interpolated ion densities (Np + 2Na) minus measured halo densities to fill the core electon density column in these data. Finally, the URAP experiment aboard Ulysses operates a radio-frequency sounder which can also distort the low-energy electron distributions. Spectra measured during sounder operations are not included in the NSSDC data submission. 5. DESCRIPTION AND FORMAT OF DATA SUBMITTED TO THE NSSDC The data provided to the NSSDC are the total, core, and halo electron densities and scalar temperatures at full instrumental time resolution, plus spacecraft position. The data submitted to the NSSDC for the SWOOPS electron experiment was replaced in its entirety in mid-May, 1999. The earlier data set did not have the several spacecraft potential and scattered light corrections and software filters and is not as reliable as the current submission.  We strongly reccommend the use of these revised calculations in all future studies. The time specified in the file is the center of each 2-minute spectrum. This time roughly corresponds to the center of the core distribution; the most appropriate time for the halo properties is roughly 30 seconds later. One file per month is provided, with a naming scheme as follows: U97244BAMELE.DAT corresponds to data starting on day 244 of year 1997. In the 21st century, years will continue to be provided in two digit format; e.g., U01244BAMELE.DAT will correspond to data starting on day 244 of year 2001. The files can be opened and read as follows: open (3, file='U97244BAMELE.DAT', status='old') c      iyr           - year c      idoy          - day of year (Jan 1 = 001) c      ihr           - hour, UT c      imin          - minute, UT c      isec          - second, UT c      sunsc         - sun-spacecraft distance, AU c      hlat          - heliospheric latitude of spacecraft, degrees c      hlong         - heliospheric (Carrington) longitude of spacecraft, deg c      den          -  total number density of electrons per cubic cm c      denc          - core electron number density per cubic cm c      denh          - halo electron number density per cubic cm c      tempe         - total electron temperature, Kelvins c      tempc		      - core electron temperature, Kelvins c      temph         - halo electron temperature, Kelvins read (3, 1) iyr,idoy,ihr,imin,isec,sunsc,hlat,hlong, >            den,denc,denh,tempe,tempc,temph 1     format(1x,i2,1x,i3,3(1x,i2),f7.4,2f7.2,6e13.5) Spectra with known bad density or temperatures, 2-d spectra, or spectra taken when the URAP sounder is operating are not submitted to the archives. 7. FURTHER INFORMATION For information on acquiring other types of data not provided to the the NSSDC, contact the 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 and electron experiments, contact Dr. Bruce E. Goldstein at the Jet Propulsion Laboratory, bgoldstein@jplsp2.jpl.nasa.gov, 818-354-7366.