| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This dataset contains count rates from the Energetic Particle Analyser
(EPA) experiment aboard the Giotto spacecraft during its flyby of
comet P/Grigg-Skjellerup during July of 1992.
The solid-state detectors of the EPA detected energetic ionizing
particles (electrons, protons, heavier ions), but no magnetic fields
or time of flight indicators were included. Thus it is difficult to
determine what type of particles are being detected, and the
calculation of the energy spectrum becomes rather complicated. It
is because of this that only count rates are included in the archive.
Parameters
==========
The dataset is broken into 4 separate tables, one for each energy
channel of the instrument. The tables list count rates per second,
averaged over a one-minute interval, for each of eight geometric
sectors.
The thresholds in each energy channel depend on species. Values for
the two species most likely to be encountered near the comet during
the flyby are given in this table:
Channel Protons H(2)O+
------- ------- ------
1 29- 46 keV 78- 115 keV
2 40- 77 97- 145
3 78- 215 144- 270
4 217-3500 280-3500
The two EPA detectors which produced the data for this archive are
oriented at a 45 degree angle to the spin axis of the space craft,
which was antiparallel to the motion of the spacecraft relative to
the comet. During the flyby data were taken so that odd-numbered
sectors came from one detector and even-numbered sectors from
another.
The following table lists the look directions of each sector,
relative to the spacecraft Z-axis. Theta is the angle to the
spin axis; phi is the rotation in the spin direction from the plane
containing the sun and Z axis. NOTE that the spin axis is equal
to the spacecraft -Z (minus Z) axis. All angles are in degrees.
Sector Detector Theta Phi
------ -------- ----- ---
1 1 135 -102.5
2 2 45 -57.5
3 1 135 -12.5
4 2 45 32.5
5 1 135 77.5
6 2 45 122.5
7 1 135 167.5
8 2 45 -147.5
Modification History
====================
This dataset was originally received as five separate ASCII files
covering successive time segments of the flyby, with averages for
all four channels appearing under headers with the start and stop
time for the averages. These files were concatenated in chronological
order, split into four files by energy channel, and reformatted. The
stop time for each average which was included in the original file has
been dropped from these files because it was invariably one minute
later than the listed start time.
Additionally, the invalid data flag in the original file (-100.000)
has been changed to -999.999 to conform with the standard null data
values in other PDS data sets.
Data
====
Each energy channel table covers the entire observation period in
one minute intervals. Sector averages are listed in the order given
above. Where data measurements were not recorded, a null value
flags the drop-out. There is also an extended data gap in all
sectors of all channels during the period 1992 July 9 UT 1810 to
July 10 UT 0420. Records with all null count rates make this
period.
There are no time gaps in the records in the data files.
NOTE: the count rates in energy channel 1 are contaminated by
sunlight in sectors 2, 5, 6, 7 and 8.
Ancillary Data
==============
None known
Coordinate System
=================
During the encounter, the spacecraft spin axis (the -Z axis) was
oriented almost in the ecliptic plane, nearly perpendicular to the
spacecraft-sun line.
In the Comet Solar Ecliptic (CSE) coordinate system, the comet is
at the origin, the X axis points toward the sun and Y is parallel
to the ecliptic so that the positive Z axis is toward the North.
Then theta is the angle from the Z axis and phi is azimuth in the
XY plane, with phi=0 for the Sun.
During closest approach, the orientation of the spin axis is:
theta = 90.0 degrees phi = 90.4 degrees (CSE)
In CSE, the look directions listed above become (alpha is the angle
to the comet-sun line):
Sector Theta Phi Alpha
------ ----- --- -----
1 46.3 -101.8 98.5
2 53.4 62.2 68.0
3 81.2 -45.3 46.0
4 112.3 50.3 53.7
5 133.7 -77.4 80.9
6 126.6 118.6 112.6
7 98.8 -133.9 133.3
8 67.7 130.5 127.0
Software
========
None
Media/Format
============
This dataset is to be included in the Giotto Extended Mission CDROM
produced by the Small Bodies Node (SBN) of the Planetary Data System.
Copies of the CD will be available to researchers from the SBN, or
to others through the National Space Science Data Center.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
Confidence level unspecified.
Review
======
Pending
Data Coverage and Quality
=========================
Unknown
Limitations
===========
Because of the lack of species discrimination, the reduction and
interpretation of these data are complex. However, there was a
third detector mounted at the same angle to the spin axis as the
second detector, which was covered with an aluminized plastic foil.
This covering absorbed low-energy protons (up to about 350 keV)
but allowed electron with energy greater than 20 keV to penetrate.
Count rates in the covered detector never exceeded those in the
neighboring detector by more than 10 percent (the typical difference
was less than 1 percent), indicating that EPA observed primarily
positive ions during the flyby.
The count rates from the covered detector are not included in this
dataset.
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