| DATA_SET_DESCRIPTION |
Data Set Overview
=================
*** NOTE WELL: The data supplier and producer failed to meet
*** his archiving obligation with these data, in
*** that a large number of science content problems
*** identified in the peer review held October 2003 were still
*** unresolved two years later, when the data were submitted
*** for archiving. Potential users should read the text of
*** the CONFIDENCE_LEVEL_NOTE carefully, and see the list of
*** unresolved problems in the 'liens_outstanding.txt' file
*** included in the documentation for this data set.
This data set contains ion and electron flux, as a function of
energy and angle, and ion time of flight measurements of ion
composition. These data were measured by the Plasma Instrument
for Planetary Exploration (PEPE) on the Deep Space 1 (DS1)
spacecraft during the DS1 encounter with comet 19P/Borrelly.
PEPE measured particles from 8 eV to 31.5 keV and over a 2.8 pi
sr range of angles. These data were acquired in order to
characterize the interaction between the solar wind and a
moderately active comet, including but not limited to the slowing
and heating of the mass-loaded solar wind, the composition,
abundance and velocity distribution of cometary heavy ions, the
location and properties of boundaries such as bow shocks and the
cometopause, and changes in the charge state of solar wind heavy
ions caused by charge transfer reactions with the neutral coma.
19P/Borrelly is a Jupiter family comet with an orbital period of
6.86 years. Earth-based spectra have identified Borrelly as
depleted in C2 and C3 relative to 1P/Halley, and is the
type-example of a class of comets which are C2 and C3 depleted,
but not significantly depleted in CN. The DS1
encounter occurred on September 22, 2001 (day of year 265), at
a distance of 1.36 astronomical units from the Sun. At that time,
Borrelly was eight days past perihelion and had, based on
several Earth-based observations, a production rate of 3.5x10E28
molecules per second. Compared to other comets encountered by
spacecraft, this is a factor of 20 to 30 less than 1P/Halley,
comparable to 21P/Giacobinni-Zinner, and four times higher than
26P/Grigg-Skjellerup [SODERBLOM2002]
The spacecraft trajectory during the encounter was nearly north-
south due to the comet's 30.3 degree inclination. Closest
approach occurred at 2001-265T22:29:32, spacecraft event time and
at a range of 2171.4 km from the nucleus. Closest approach was
near the comet-Sun line, at a phase angle of 4.2 degrees. At the
time of closest approach, the spacecraft's position and velocity
relative to the nucleus were (-693.6,-1866.7,-865.5) km and
(2.40,6.17,-15.21) km/s in J2000 coordinates, and a unit vector
pointing from the nucleus to the Sun was (-0.2535,-0.8880,
-0.3838). The spacecraft position may be calculated by assuming
the spacecraft velocity was constant. This approximation is
accurate to within 150 km and 1.2E-4 time the spacecraft-comet
range at all times within 24 hours of closest approach. More
accurate information on the trajectory may be obtained from the
SPICE spk kernels. During the encounter, there were numerous
spacecraft turns, to track the nucleus, observe guide stars,
orient the spacecraft for optimal PEPE observations on the
outbound leg and to turn to downlink data to Earth. A more
complete description may be found in [RAYMAN2002] and the
exact orientation of the spacecraft may be found in the relevant
SPICE ck kernels.
Processing
==========
The PEPE data have been processed by:
(1) Extracting the raw PEPE data from the packetized telemetry
files.
(2) Converting data from 8-bit data numbers to 16-bit counts
(see the Limitations section of CONFIDENCE_LEVEL_NOTE
in this file) and conversion of housekeeping values into
physical units.
(3) Converting time tags from spacecraft clock time to
spacecraft clock time, using NAIF toolkit software and
SPICE kernels.
(4) Reformatting data into the PDS archive format.
Parameters
==========
Data Sampling:
PEPE data are acquired by simultaneously integrating counts in
16 azimuthal pixels. Counts in 16 elevations and 128 energies
are collected sequentially by stepping the voltage applied to the
instrument's deflector plates (elevation) and electrostatic
analyzer (ESA) plates (energy.) A complete set of elevations is
collected at each energy, before lowering the ESA voltage to the
next step.
Offset times have been added to the data to indicate when,
relative to the start of each measurement, a specific elevation
and energy was sampled. However, all PEPE data products are
collapsed by summing over a number of samples, to reduce data
volume. The offset time gives the start of the first integration
in the collapsed data. The following table illustrates the order
in which samples are integrated.
Table 1. PEPE Sampling Order
-----------------------------------------------------------------
Energy |Elevation 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
-----------------------------------------------------------------
0 | 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1 | 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
2 | 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
3 | 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
... | ...
-----------------------------------------------------------------
In, for example, the electron (ELC) data, samples are collapsed
by summing together blocks of four elevations and two energies.
One collapsed value would be the sum of elevations 0 to 3 and
energies 0 and 1 are summed. I.e. the 0th, 1st, 2nd, 3rd, 16th,
17th, 18th and 19th samples measured by the instrument. The
offset time in the data would be the time of the start of the
0th integration. The offsets in the ion 'singles' and
'logicals' are similarly affected by collapsing the data.
Data collapse and integration times:
The instrument's data rate is controlled by (1) changing the
method of collapsing data and (2) varying the integration time
at each elevation and energy. During the Borrelly encounter, only
one collapse mode was used. PEPE was operated at two data rates
during the encounter. From the start of the observations until
2001-265T10:23:05, the instrument was in a 50 bps mode, with an
integration time of 572.4 ms, requiring 1302 seconds per full
measurement. After 2001-265T10:23:05, the PEPE was operated in a
1 kbps mode, with an integration time of 28.62 ms, requiring 65.5
seconds per full measurement.
Energy range:
During the Borrelly encounter, PEPE used one of two look up tables
to determine the range of energies sampled. The number of energy
steps per measurement was the same in both energy tables. The
spacecraft's ion propulsion system (IPS) produces a extremely
high flux of low energy, heavy ions. For instrument health
reasons, PEPE did not sample energies below 15 eV when the IPS was
on. Instead, the instrument's ESA was set to zero volts for these
energy steps. When the IPS was off, the ESA was set to zero for
the final 1/16 of the energy steps, providing data on energies
down to 8 eV as well as background measurement. The following
table gives the times at which these energy tables were used.
Table 2. PEPE Energy Table Use
------------------------------------------------------------------
Start time End time Energy Table
2001-262T15:55 2001-265T10:23 IPS blocked
2001-265T10:23 2001-265T11:27 Full range
2001-265T11:27 2001-265T12:25 IPS blocked
2001-265T12:25 2001-266T00:20 Full range
2001-266T00:20 2001-266T10:44 IPS blocked
Data
====
The PEPE data consist of seven data products, each archived in
separate files. These products are electron spectra, selected
housekeeping values, ion 'singles' spectra, ion 'logicals'
spectra, ion 'mass over charge' spectra and ion time-of-flight
mass spectra. The contents of these products is described below.
Each file contains all measurements with a start time on the
specified day. File names are of the form xxxyyddd.dat where
xxx = elc Electron data
hsk Housekeeping data
ion Ion singles data
log Ion logicals data
mq Ion mass over charge data
tof Ion time of flight data
yy = 01 Year
ddd = Day of year
_t = A=ASCII, B=binary
The PEPE data are obtained by integrating counts at 16 azimuths,
16 elevations and 128 energies. The full volume of data produced
by the instrument exceeds the maximum, 1 kbps rate at which the
spacecraft collects data. Azimuth/elevation/energy samples are
collapsed within the instrument to fit within this limit. In the
archived data files, azimuth, elevation and energy steps always
refer to the collapsed data products, not the full 16x16x128
range collected within the instrument.
All data products use the same collapse in energy, from 128 to
64 steps. Each uncollapsed energy step is centered on an energy
of 32675*(4096**(-N/119)) eV. The following table gives the
conversion between collapsed energy step number and the observed
particle energy per charge.
Table 3. Energy step number and
------------------------------------------------------------------
Energy Energy Note
Step [eV]
------------------------------------------------------------------
0 31572.1
1 27453.1
2 23871.5
3 20757.1
4 18049.1
5 15694.3
6 13646.8
7 11866.4
8 10318.3
9 8972.1
10 7801.6
11 6783.8
12 5898.7
13 5129.2
14 4460.0
15 3878.1
16 3372.2
17 2932.2
18 2549.7
19 2217.1
20 1927.8
21 1676.3
22 1457.6
23 1267.4
24 1102.1
25 958.3
26 833.3
27 724.6
28 630.0
29 547.8
30 476.4
31 414.2
32 360.2
33 313.2
34 272.3
35 236.8
36 205.9
37 179.0
38 155.7
39 135.4
40 117.7
41 102.4
42 89.0
43 77.4
44 67.3
45 58.5
46 50.9
47 44.2
48 38.5
49 33.5
50 29.1
51 25.3
52 22.0
53 19.1
54 16.6
55 14.5 0 eV in IPS blocked energy table
56 12.6 0 eV in IPS blocked energy table
57 10.9 0 eV in IPS blocked energy table
58 9.5 0 eV in IPS blocked energy table
59 8.3 0 eV in IPS blocked energy table
60 0.0
61 0.0
62 0.0
63 0.0
Electron Spectra:
These data are the counts of electrons, in four azimuthal, four
elevation and 64 energy bins. These bins are
Table 4. Electron Azimuths
--------------------------------------------
Azimuth number Minimum angle Maximum angle
--------------------------------------------
0 0 90
1 90 180
2 180 270
3 270 360
Table 5. Electron Elevations
----------------------------------------------
Elevation number Minimum angle Maximum angle
----------------------------------------------
0 -45 -22.5
1 -22.5 0
2 0 22.5
3 22.5 45
Selected housekeeping data:
TBS
Ion Singles:
These data are counts of ions, measured in the time-of-flight
system's start detectors. These counts are accumulated
independently of the TOF measurement, and regardless of whether
or not the start led to a valid TOF event. This gives them a
higher sensitivity and lower dead time than the TOF measurements
but also a high background. These data are returned in eight
azimuthal and eight elevation bins, and the azimuthal bins have
a variable pixel size. Tables 6 and 7 give the angular coverage
of these bins.
Table 6. Ion Singles Azimuths
--------------------------------------------------------
Azimuth number Minimum angle Maximum angle Width
--------------------------------------------------------
0 340 350 10
1 350 360 10
2 0 10 10
3 10 20 10
4 22.5 67.5 45
5 67.5 157.5 90
6 157.5 247.5 90
7 247.5 340 90
Table 7. Ion Singles Elevations
----------------------------------------------
Elevation number Minimum angle Maximum angle
----------------------------------------------
0 45 33.75
1 33.75 22.5
2 22.5 11.25
3 11.25 0
4 0 -11.25
5 -11.25 -22.5
6 -22.5 -33.75
7 -33.75 -45
Ion Logicals:
These data are counts of various single events. The PEPE TOF
system uses these events to start and stop TOF measurements and
to determine if a TOF event is valid. Ion logicals are primarily
used to diagnose the performance of the TOF system, and to provide
additional data for dead time corrections. These data are not
azimuthally resolved and are collapsed into two elevations:
Table 7: Ion Logical Elevations
-----------------------------------------------
Elevation number Minimum angle Maximum angle
----------------------------------------------
0 45 0
1 0 -45
The logicals are:
Coarse starts: All start counts measured in the coarse azimuth
pixels, AZ 4-7 (whether or not the start resulted
in a valid TOF event).
Fine starts: All start counts measured in the fine azimuth
pixels, AZ 0-3 (whether or not the start resulted
in a valid TOF event).
Stops: All stop counts, whether or not they were
preceded by a start event.
Resets: All start events which reset, i.e. which were not
followed by a stop event within 1536 ns.
Ion Mass-Charge Ratio:
These data are counts of TOF events, selected on the basis of
their time-of-flight (i.e. all events with a TOF in a specified
range.) This largely (but not entirely) separates the data
into energy spectra of species with a mass to charge ratio within
a specific range. The TOF bins were selected so that MQ bin 0
would contain exclusively protons, MQ bin 1 would contain
primarily He++, and so that the remaining 13 bins would span
the TOF range with logarithmic widths. These data are collapsed
over all azimuths and elevations. The TOF bins used are:
Table 8. MQ Bins
---------------------------------------------------------
MQ number Minimum TOF Maximum TOF
0 48 88
1 89 105
2 106 125
3 126 150
4 151 179
5 180 214
6 215 256
7 257 307
8 308 367
9 368 438
10 439 524
11 525 627
12 628 750
13 751 896
14 897 1071
Ion Time of Flight:
These data are time-of-flight mass spectra, summed over all
energies, elevations and azimuths. TOF bins 48 to 1071 were
returned in these data. The ions' time-of-flight is nominally
0.75 ns * TOF bin number. Post-launch experiments with the PEPE
prototype were more consistent with 0.82 ns * TOF bin number.
The path length between the start foil and the stop detector is
81.4 mm.
Ancillary Data
==============
No ancillary data are included in this data set.
Coordinate Systems
==================
DS1 spacecraft coordinates are defined so that the +Z axis is
is aligned with the IPS direction of thrust and the boresight of
MICAS (Miniature Integrated Camera And Spectrometer) instrument.
The spacecraft's high gain antenna and the PEPE instrument's
direction of zero elevation and azimuth are aligned with the
+X axis. The spacecraft's solar arrays are extend along the Y
axis.
The PEPE data are binned by instrument elevation and azimuth. In
spacecraft coordinates, a unit vector in the direction azimuth (a),
elevation (e) is {cos(a)*cos(e),sin(e),sin(a)*cos(e)}.
|
| CONFIDENCE_LEVEL_NOTE |
Review
======
These data have been reviewed by the instrument team and are of
the highest quality that can be generated at this time. Science
results based on some of these data have been submitted to several
journals (GRL and Icarus).
External Peer Review - CAUTION
==============================
The PDS external peer review for these data was held in October 2003.
A number of liens were placed on the data based on the science content
and, in particular, aspects of the documentation. After two years many
of these liens remain either unresolved by the data preparer, or of
uncertain status. The decision was made to archive these data anyway
in October of 2005, with the list of liens included as part of the
data set documentation.
In addition, the text above, describing the data set, and the text
in the instrument catalog file both contained several references to
apparently non-existent publications. These references have been
removed. The YOUNGETINPRESS reference which remains has yet to be
published as of this writing (October 2005) and consequently is also
suspect, but has been left in place.
In the event that a fully corrected and documented version of these
data are ever received, it will be archived as Version 2 of this data
set.
Data Coverage and Quality
=========================
These data contain several, brief gaps. Most are associated with
instrument mode (integration time) or energy sweep change. Table
M gives the start and end times of these gaps.
Table M. Gaps in coverage
------------------------------------------------------------------
Start time End time
------------------------------------------------------------------
2001-264T00:51:03 2001-264T06:16:41
2001-264T17:29:39 2001-264T17:51:22
2001-265T10:08:14 2001-265T10:23:04
2001-265T11:27:07 2001-265T11:30:04
2001-265T12:25:26 2001-265T12:28:04
2001-266T00:20:11 2001-266T00:22:28
In addition, there are two measurements with missing MQ, logical
and TOF products. These are the measurements with start times
at 2001-265T07:14:36 and 2001-265T10:32:52.
Limitations
===========
To reduce data volume, counts as measured on spacecraft are
converted from 16-bit to 8-bit integers. This mapping is linear
for low values and approximate logarithmic at high count rates.
The data numbers are converted back into 16-bit integers on the
ground. This introduces an approximately -0 to +3 percent
uncertainty in the actual number of counts. Users of these data
should assume an uncertainty of sqrt(N + 0.015*N**2) rather than
the usual, statistical uncertainty of sqrt(N).
The PEPE time-of-flight spectra are subject to repeated pattern
noise. The instrument computes time of flight using a clock with
12 nanosecond ticks and an electronic vernier which provides 16,
approximately 0.75 picosecond sub-ticks within each clock cycle.
Due to irregularities in the
electronic vernier, the time-of-flight spectra contain repeated
pattern noise, with a 16 bin period.
The electron measurements contain a high background at energies
above approximately 3 keV. The cause of this background is not
completely understood, but is related to solar UV photons reaching
the electron detector. All electron data above 3 keV should be
considered suspect.
TOF run at 11 kV not 8 or 15 (calibration, low + ion yield,
high scattering) electrons with IPS on (s/c potential) TOF
background with IPS on.
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