Version 1.0 of the Pioneer 11 Data Set The Pioneer 11 data sets contains five types of data: 1. 'REDR' and EDR data 2. 'Spectral' data 3. 'Summary' (high resolution) parameter data 4. Hourly averaged parameter data 5. Trajectory data. The summary and hourly averaged parameter data were derived from fits to solar wind spectra measured by the ARC Plasma Analyzers aboard the Pioneer 10 and 11 spacecraft. The 'Block 3 Least Squares' processing scheme was used to obtain plasma parameters. This involves least squares fits of a convected Maxwellian to solar wind spectra, roughly analogous to the 'fit' parameters from the Voyager spacecraft. Velocities, densities, and temperatures from this scheme should be fairly reliable except for possible anomalies in the measurement of density described below. For historical reasons, data from this scheme only include measurements from Detector B of the plasma analyzer. This data was used to create the 'summary' (high resolution) data set. Points from this 'summary' data set were then averaged to produce the hourly averaged data set. Format These files are written in ASCII in an 80-column format for ease of translation. Each file contains a header followed by data records. Each data record contains the spacecraft ID (SCID), year, day of year, and time of day in hours, followed by the bulk velocity, number density, temperature, and where appropriate, the flow angles. Co-ordinate systems and units Speeds are given in km/s, number densities are given in #/cc, temperatures are given in K, and flow angles are given in degrees. Prior to day 31 of 1989, all directions are given in a spacecraft-centered ecliptic system, in which the E/W angle is positive in the direction of planetary rotation while a N/S angle of zero means that the solar wind flow is parallel to the plane of the ecliptic. NOTE: unless the spacecraft is in the plane of the ecliptic, this is different from an RTN system, in which a N/S angle of zero means that the solar wind flow is radial. After day 31 of 1989, directions are given in an RTN system. This inconsistency will be eliminated in a future version of the data set. Peculiarities of the Pioneer 11 Data Set For some unknown reason, the plasma analyzer aboard Pioneer 11 ceased opperation temporarily between day 102 of 1975 and day 341 of 1977. Unfortunately, this period encompasses the spacecraft's first excursion to heliographic latitudes greater than 10 degrees. Other instruments aboard the spacecraft continued to function and the plasma analyzer resumed normal operation on day 341 of 1977. After day 151 of 1992, the RTGs aboard Pioneer 11 no longer produced sufficient power to opperate the plasma analyzer and the instrument was turned off. Uncertainties Preliminary tests with simulated spectra suggest that the uncertainties in the Pioneer 11 parameters are: Parameter Uncertainty _________________________________________________________________ V 3-5 km/s N Internally consistent to 20%. Simulations also suggest an absolute accuraty of 20%, but N seems to be 50% higher than comparable measurements from Voyager 2. T 20% for T > 10,000 K Approximately 5000 K for 5000 K < T < 10,000 K Unreliable for T < 5000 K Flow Angles Accuracy remains to be determined. It should be noted that these tests are still underway! Until those tests are complete, the absolute accuracy and sensitivity limitations of densities and temperatures from Pioneer 11 should not be regarded as well-determined as those from Pioneer 10. Anomalies in the Pioneer 10 and 11 density measurements As of this release, there may be three anomalies in density measurements from Pioneer 10 and 11. These should be kept in mind if detailed comparisons are to be made with data from other spacecraft. 1. As mentioned above, values for N measured at Pioneer 10 and 11 seem to be 50% higher than comparable measurements from Voyager 2, after accounting for the radial expansion of the solar wind. This may reflect uncertaintainties in the calibration of these spacecraft. It remains to be determined which spacecraft is more 'correct'. 2. Tests involving simulated spectra suggest that the Block 3 Least Squares processing scheme may consistently underestimate densities when densities are high. This should not be important at heliocentric distances > 10 AU, where the solar wind density is typically < 1, but there is some suggestion that reported densities could be as much as 20% too low early in the missions, when the spacecraft were closer to the Sun and actual densities were > 10. Work is in progress to understand, and if necessary to correct, this anomaly. 3. With the existing data production, the plasma analyzers aboard the Pioneer spacecraft begin to suffer from sensitivity problems when N drops below 0.003. This should not be an issue at Pioneer 11, which spent most of its time at heliocentric distances less than 35 AU. REDR and EDR data REDR (Reformatted EDR) and EDR (Experiment Data Record) data is a low-level product, very close to raw telemetry, that contains segments of data from the plasma analyzers interlaced with engineering data. This data is stored at Ames for production purposes. Because it can only be read with specialized software there is no plan to make this data available for general distribution. All of the information it contains is available in the 'spectral data' descibed below. Spectral data The spectral data set contains spectra from the ARC plasma analyzer, along with associated engineering data, in a compact and easily read format. Every spectrum has been stored; no attempt has been made to 'filter out' noise, bad, or partial spectra. Because it reflects the full resolution of the plasma analyzer (approximately 1 spectrum ever 13 minutes early in the mission) this may be the desired data set for use in detailed studies of waves, discontinuities, and the microscopic physics of the solar wind, but it must be used with caution. Due to its size, this data set is stored offline. For more information, contact Paul R. Gazis Summary (High Resolution) Data Set [Image] NOTE: This data has not yet been installed on this system! If you require high-resolution data, you should contact Paul R. Gazis The summary data set contains individual fits to valid solar wind spectra from the plasma analyzers. Velocities have been corrected for abberation due to the motion of the spacecraft and all times are in spacecraft event time (SCET). After the mid-1970s, every point was processed. Earlier in the mission, when data rates were higher, only a few points could be processed each hour with the limited computational resources that were available in the early 1970s. Work is in progress to reprocess all of the data from the early phases of the mission. Until this can be accomplished, the hourly averages will be the preferred data set for most studies of the solar wind. Hourly Averaged Data Set Hourly averaged data files were created by averaging every valid parameter in each hour. This is the preferred data set for use in most studies of the solar wind. Trajectory Data The trajectory data were derived from actual SEDR files prior to mid-1988 which were extrapolated to the end of 1999. NOTE: Due to imprecisions in the extrapolation, it is only accurate to approximately 100,000 km. This is more than adequate for studies of the heliosphere. Those who require greater accuracy should seek data from either the National Space Science Data Center (NSSDC) or the Space Physics Data System (SPDS). These files are written in ASCII in an 80-column format for ease of translation. Each file contains a header followed by data records. Each data record contains the spacecraft ID (SCID), year, day of year, and time of day in hours, followed by the Cartesian state of the spacecraft in ecliptic co-ordinates where the x-axis is directed towards the first point in Ares while the z-axis points north. For more information, contact: Dr. Paul R. Gazis, SJSU Research Associate phone: (415) 604-5704 fax: (415) 604-6779 email: gazis@arwen.arc.nasa.gov ------------------------------------------------------------------------------- [Go to] Parent Directory [Go to] ARC Pioneer Plasma Group Home Page last modified 28 July 1995.