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
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