SWOOPS/Ion - 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 POSITIVE ION EXPERIMENT TABLE OF CONTENTS 1. OVERVIEW OF THE SWOOPS EXPERIMENT 2. INTRODUCTION TO THE SWOOPS ION EXPERIMENT 3A. INSTRUMENT OPERATION-SUMMARY 3B. INSTRUMENT OPERATION-DETAILED DESCRIPTION 4. DATA REDUCTION ALGORITHMS 5. DESCRIPTION AND FORMAT OF DATA SUBMITTED TO THE NSSDC 5.A FORMAT OF HIGH RESOLUTION DATA 5.B FORMAT OF HOURLY AVERAGE DATA 6. PERIODS OF BAD ATTITUDE OR SUN SENSOR PROBLEMS 7. LONG TERM ERRORS IN ION FLOW DIRECTIONS 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 experiments that operate asynchronously. For this reason, the data from the experiments are submitted separately to the NSSDC. This document describes the positive ion analyzer and the data submitted to the NSSDC for that analyzer; the electron experiment is described in a comparable document that accompanies the electron data. The experiment was designed as a solar wind investigation, and the results from the positive ion part of the experiment at Jupiter were generally not useful because of field of view problems; Jovian magneosheath and magnetosphere data (1992 033 17:00 to 1992 048 02:00) are therefore not submitted to the NSSDC. 2. INTRODUCTION TO THE SWOOPS ION EXPERIMENT The SWOOPS ion analyzer measures 3-d velocity space distributions of positively-charged ions. In its normal solar wind mode, the instrumental energy range is 0.25 to 12 keV/Q; active energy tracking allows for fine energy spacing within a smaller range which is ample for characterization of the thermal proton and helium distributions in most cases. The analyzer does not make an explicit ion species determination; separation of protons from doubly-charged helium is enabled by the fact that the two species have similar convection speeds. During periods of spacecraft nutation only the bulk speed and density are reliable, individual velocity components and temperatures are invalid. The beginning of scientifically useful SWOOPS data is at Day 322, 00:59 of 1990; at that time the spacecraft was already nutating; nutation ceased as of Day 351, 22:00, 1990. As Ulysses approaches the Sun, nutation may resume during periods when active attitude control is not available (active attitude control requires a continuous uplink). 3A. INSTRUMENT OPERATION-SUMMARY Each spectrum takes 2 minutes to accumulate, but telemetry takes longer. A spectrum is telemetered to the ground in 4 minutes at highest bit rate, at lower spacecraft bit rates the time is greater. Depending on the mission phase, the data may all be high time resolution, may be partially high time resolution (some 4 minute sampled data, and some 8 minutes sampled data), or may be sampled at rates even slower during certain occasions. Depending on the bit rate, every 5th (highest bit rate) or 9th spectrum (all slower bit rates) is obtained with half the energy resolution of the other spectra. 3B. INSTRUMENT OPERATION-DETAILED DESCRIPTION The positive ion analyzer is provided with a 200-level high voltage supply to cover a range of ion energies from 255 eV/q to 34.4 keV/q. Level spacings are 2.5%, and are used in a variety of ways. The normal SWOOPS mode of operation consists of three types of modes, two that deal with protons and alpha particles, and one (not described here) that deals with heavy ions. The two types of measurement modes used for measuring protons and alpha particles are the Search (S) mode and T (track) mode. The instrument first does a coarse energy scan (S mode) consisting of 40 steps (every 4th step in the lowest 160 of the instrument's 200 available levels). Having located the solar wind flux peak, the instrument then does several T scans in the region around the peak using every other energy step. To obtain increased energy resolution during periods when the solar wind is not changing speed, the instrument was designed so that T scans alternate using the even- and odd-numbered steps of the voltage supply. The data provided to the NSSDC are based upon individual scans; the alternating even (E) and odd (O) scans have not been combined. During the highest bit rate operation one S scan is followed by eight T scans, at all other data rates one S scan is followed by four T scans. Differences between estimates of solar wind parameters based on lower energy resolution data (S mode) and neighboring T mode data are at times noticeable in plots as S mode outliers in the velocity and temperature values. Generally speaking, the alternating types (O and E) of T mode do not lead to apparent effects in the reduced parameters. However, during exceptionally cold periods, such as within a Coronal Mass Ejection at distances significantly beyond the orbit of the Earth, upon rare occasions an O/E mode alternation in the density and direction of flow is apparent in the reduced data. SWOOPS heavy ion data are not submitted to the NSSDC because the instrument does not have a mass separation capability or coincidence detection for noise suppression. Data from the SWICS experiment are better suited for heavy ion investigations. 4. DATA REDUCTION ALGORITHMS Plasma parameters are calculated by numerical integration of velocity-weighted ion distributions over an E/q range chosen to include the thermal proton and alpha-particle populations. Under extremely hot conditions, there can sometimes be some overlap between these populations. Additionally, during periods when the solar wind temperature is exceptionally low the experiment can not properly measure the temperature. Care has been taken to estimate the instrument background from channels that do not contain data, and the effects of background have been removed from the integration. The velocity space resolution of the experiment is better in the energy dimension than in the angular dimensions. The proton temperature has been estimated in two different ways, one of which will sometimes lead to an overestimate of the temperature and one of which will sometimes lead to an underestimate. T-large is the integral of the distribution in three-dimensional velocity space over all energy channels and angle bins that are statistically above noise. The criterion for noise determination is based upon the estimation of the total contribution from a shell that is spherically symmetric in velocity space. T-large has the drawback that at times when the solar wind is cold the angular responses of the instrument channels are as wide or wider than the beam, and the temperature is overestimated. T-small is estimated by summing over angle the observations at a fixed energy. The moments of the resulting one dimensional plasma spectrum are then summed and the resulting RR (radial) component of the temperature tensor is used as the estimate of proton temperature. Additionally, to avoid contamination from alpha particles, no channels further in velocity space from the proton peak than the minimum in flux between the proton and alpha particles peaks are used in the T-small estimate. T-large and T-small will generally bracket the true temperature. However, for very cold plasma, T-small may not be a true minimum. In the analyses already described, when performing the numerical integration over velocity space it is assumed that the instrument response within a given channel is uniform and that the viewing area of the channel is uniformly filled with plasma. A least squares fitting procedure to a bi-Maxwellian distribution that includes the instrument response has been to improve estimates under difficult conditions (very cold plasma). However, this procedure can not be used for routine data reduction because of computer time requirements. Persons desiring specialized analyses of particular time periods should request the needed data directly from the SWOOPS team (see section 8 below). In addition to the issues of temperature determination already discussed, there are also small systematic uncertainties in the velocity components. These are due to such effects as uncertainty in the accuracy of alignment on the spacecraft, variation in gains between channeltrons, etc. The velocity component data should not be used for studies of large scale, long term, solar wind deflection. The data are generally suitable for all other studies. The data have been corrected for spacecraft velocity. The Odd and Even mode scans start at odd or even energy steps of the instrument, and during a scan the energy channel is increased by two steps for each observation. To improve the temperature resolution of the experiment we have also produced combined parameters from adjacent Odd and Even scans. If the peak shifts to that the peak of adjacent Odd and Even spectra are not contiguous in the scan, then a combined spectra is not produced. Since there are sometimes considerably less data in the combined than in the O/E mode, the high resolution data provided to the NSSDC are the O/E mode. The hourly averages are based upon the combined spectra. Users are CAUTIONED that the hourly averages are based on combined mode (higher energy resolution spectra) will typically provide temperature estimates a bit lower (depends on distance from the Sun) than hourly averages of the O/E mode high resolution spectra. Spacecraft velocities are removed from all Ulysses data; i.e., velocities are defined with respect to the Sun. 5. DESCRIPTION AND FORMAT OF DATA SUBMITTED TO THE NSSDC The data provided to the NSSDC are the fluid (mass averaged proton and alpha particles) velocity components, proton number density, alpha particle number density, and an upper and lower limit on proton temperature. Two types of data are provided: high resolution data and hourly averages. Definition of Solar Heliospheric RTN coordinates: R is a unit vector pointing from the Sun to the spacecraft. T is Omega x R, where Omega is a unit vector along the Sun's rotation axis. N = R x T. 5.A FORMAT OF HIGH RESOLUTION DATA The time specified in the file is 3.5 spins after the beginning of the acquisition of a spectrum. For T mode data, the peak of the proton flux is typically found at energies sampled during spins 3 and 4. For S mode data the time at which the peak is observed is variable and depends on the solar wind speed, hence the time in the file does not precisely reflect the time the observation is made. open (3, file='SwoopsIon', status='old', readonly, + carriagecontrol='list') c      iyr           - year c      idoy          - day of year (Jan 1 = 001) c      ihr           - hour c      imin          - minute c      isec          - second c      rau           - sun-spacecraft distance, AU c      hlat          - heliosperic latitude c      hlong         - heliospheric longitude c                      (Carrington longitude of spacecraft) c      densp         - proton number density per cubic centimeter c      densa         - alpha number density per cubic centimeter c      tlarge        - proton temperature, Kelvin c      tsmall        - proton temperature, Kelvin c      vr, vt, vn    - plasma velocity (km/sec) in heliospheric c                      RTN coordinates in the solar system frame c      iqual          - condition flags read (3, 1) iyr, idoy, ihr, imin, isec, + rau, hlat, hlong, densp, densa, tlarge, tsmall, vr, vt, vn,iqual 1      format (1X,I2,1X,I3,3(1X,I2),F7.4,2F7.2, + 2(1X,F10.5),1X,2F10.0,3(1X,F7.1),1X,I4) The variable iqual is set to 0 for "good" data. "Good" means that there was nothing detectable wrong with the data except that in some cases the temperature is so high that the proton observations overlap the alpha particle observations. If iqual=1, there is something questionable about the data. Bit errors that have automatically been corrected by the program, anomalies in the data that are sometimes detected by the program when the data are too cold for proper reduction, spectra for which some missing data have been zero filled, etc., all have iqual=1. Spectra for which the alpha particle portion of the analysis failed are not included in the data submitted to the NSSDC. In general, users should restrict the use of data to iqual=0. However, data that are bad can often be ascertained by inspection when plotted in a time series, so that for studies of individual events the iqual=1 data can also be useful. 5.B FORMAT OF HOURLY AVERAGE DATA The hourly average data are computed from the high resolution data excluding points for which iqual=1. The format of the hourly data is identical to that of the high resolution data except that the iqual word is not written to the file. read (3, 9) iyr, idoy, ihr, imin, isec, + rau, hlat, hlong, densp, densa, tlarge, tsmall, vr, vt, vn 9      format (1X,I2,1X,I3,3(1X,I2),F7.4,2F7.2, + 2(1X,F10.5),1X,2F10.0,3(1X,F7.1)) The time intervals for an hourly average are from the beginning to the end of an hour, e.g., from 01:00 to 02:00, with the time in the data record being at the center of the interval, e.g., 93 125 01 30 00. 6. PERIODS OF BAD ATTITUDE OR SUN SENSOR PROBLEMS During certain periods the spacecraft was undergoing reorientation, or the sun sensors on the spacecraft were being switched and the spacecraft orientation was unknown.  For such data, only the speed and temperatures are reliable; the velocity components are unreliable.  A list (not claimed to be complete) of times during which such problems are present is: 90 348 1800 90 349 0300 SUNSP   SUN SENSOR PROBLEM 90 353 1500 90 353 1800 SUNSP   (FOLLOWED BY DATA GAP UNTIL 354 0230) 91 030 0000 91  30 0900 SUNSP   SUN SENSOR PROBLEM 93 319 1100 93 319 1957 BADAT   SPACECRAFT ATTITUDE MANUEVER 94 052 1155 94 052 1700 BADAT   SPACECRAFT ATTITUDE MANUEVER 7. LONG TERM ERRORS IN ION FLOW DIRECTIONS SWOOPS ion solar wind flow directions have long term (months) systematic errors of up to about 2 degrees. For studies involving shorter time periods in which only variations rather than absolute values are needed, this error may be ignored. For longer term variations, the user might wish to assume that the average direction of flow from the Sun is radial (as mentioned above, spacecraft velocity has been removed from the data), and apply a high pass filter to the data. 8. 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.