| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Imperial College Anisotropy Telescope
-----------------------------------------
(Last updated: 17 March 1995)
This document describes the Anisotropy Telescopes, the
data they provide, and the format of the data files
provided by Imperial College to the Ulysses Data System.
NB. Please take careful note of the following points:
(1) THE INFORMATION PROVIDED IN THIS FILE CORRECTS AND UPDATES
INFORMATION PRESENTED IN [SIMPSONETAL1992A] In particular,
the Anisotropy Telescope pointing directions were given
incorrectly in the Astronomy and Astrophysics paper, and the
energy channel widths quoted there have since been updated.
(2) See the Ulysses Data System guidelines for restrictions on
the use of this data.
1. The Anisotropy Telescopes
----------------------------
The Anisotropy Telescope (AT) experiment is a two-telescope
device. The telescopes are nominally identical with Telescope
T1 pointing at 60 degrees to the spacecraft spin axis (i.e. in
the Earthward looking hemisphere) and telescope T2 pointing at
145 degrees (i.e. in the anti-Earthward looking hemisphere).
They have a full-cone opening aperture of 70 degrees and a
geometric factor of 0.7 cm^2 sr. Together they provide 80%
coverage of the full sky. The most significant gap in the
coverage is a cone with a half angle of 25 degrees centered on
the Earthward pointing line along the spacecraft spin-axis.
There is also a 5 degree gap in coverage between the two
telescopes.
2. The full AT Data set
-----------------------
Spin-averaged and sectored data in a variety of energy channels
is produced by the ATs, as follows:
Channel Energy range Energy Range Sectored /
number /MeV (protons) /MeV (alphas) Spin-averaged
1 0.7 - 0.9 2.3 - 2.5 Spin-averaged
2 0.9 - 1.3 2.5 - 2.7 Spin-averaged
3 1.3 - 2.2 2.7 - 3.0 Spin-averaged
4 2.2 - 3.6 - Spin-averaged
5 3.6 - 6.5 24.0 - 28.0 Spin-averaged
6 0.7 - 1.3 2.3 - 2.7 Sectored (8)
7 1.3 - 2.2 2.7 - 3.0 Sectored (8)
8 2.2 - 3.6 - Sectored (8)
9 3.6 - 6.5 24.0 - 28.0 Sectored (8)
10 - 3.0 - 7.5 Spin-averaged
11 - 7.5 - 12.0 Spin-averaged
12 - 12.0 - 26.0 Spin-averaged
13 - 3.0 - 7.5 Sectored (4)
3. AT data provided to the Ulysses Data System
----------------------------------------------
The AT data provided in these files is a subset of the total
data set which is held at Imperial College.
The format of these files is as follows:
There is one data file per day. Each data file contains data
records for every ten minute interval in the day, except
during the Jupiter encounter (1992, days 25 to 48 inclusive)
where data at 256s resolution is provided.
Each record contains 17 fields of information, the first
being a time-stamp referring to the centre time of the data
contained in the remaining 16 fields. The definition of these
fields, including their FORTRAN format, is given below.
N.B. Fluxes quoted are from telescope T1, which points at 60
degrees to the spacecraft spin axis.
Field FORTRAN Definition
Format
1 a24 Time in format yyyy-mm-ddThh:mm:ss.sssZ
2 1pe10.4 Flux from channel 1
3 1pe10.4 Flux from channel 2
4 1pe10.4 Flux from channel 3
5 1pe10.4 Flux from channel 4
6 1pe10.4 Flux from channel 5
7 1pe10.4 Flux from channel 10
8 1pe10.4 Flux from channel 11
9 1pe10.4 Flux from channel 12
10 1pe10.4 Flux from channel 7, sector 1
11 1pe10.4 Flux from channel 7, sector 2
12 1pe10.4 Flux from channel 7, sector 3
13 1pe10.4 Flux from channel 7, sector 4
14 1pe10.4 Flux from channel 7, sector 5
15 1pe10.4 Flux from channel 7, sector 6
16 1pe10.4 Flux from channel 7, sector 7
17 1pe10.4 Flux from channel 7, sector 8
Each record can be read using the FORTRAN format
'(i4,4(1x,i2),1x,f6.3,16(1x,1pe10.4))'
or
1x,a24,16(1x,1pe10.4)
[please note that time-stamp and record format have been
changed by the PDS/PPI Node for the sake of consistency with
other data sets found on this volume]
3.1 Definition of flux
----------------------
in this table, flux means differential flux in units of
(cm^2 sr s MeV)^-1 (NB. per MeV, not per MeV per nucleon).
The fluxes have been computed by dividing the observed count
rates by the geometric factor and the energy channel width.
The energy channel width depends on species. Where a channel
is sensitive to alpha particles and protons (i.e., channels
1, 2, 3, 5, 6, 7 and 9), the proton width has been used since
normally the alpha contribution is insignificant. However,
sufficient information has been given above to reconstruct the
channel count-rates if necessary.
All AT channels exhibit a 'background' which may originate from
electronic noise or from activation of the material of the
telescopes or of the spacecraft. These background fluxes have
been subtracted from the data provided in these files. Channel
3 and channel 7 are the channels least affected by background
and it is for this reason that channel 7 has been selected to
provide the sectored data included in these files.
The data have also been 'despiked', although it is not
guaranteed that all spikes have been removed.
3.2 Definition of sectors
-------------------------
All sectored channels except channel 13 have eight 45 degree
sectors. Channel 13 has only 4 sectors. Sector 1 of channel
13 is a 90 degree sector equivalent to sectors 1 & 2 of any of
the other sectored channels. Similarly, sector 2 of channel
13 is equivalent to sectors 3 & 4 of any other sectored
channel, etc.
The sectoring scheme is defined relative to the 'sun crossing
line'. The sun-crossing line is the line in the plane defined
by the Sun, the Earth and Ulysses, which is perpendicular to
the Ulysses-Earth line and which makes an angle of less than
90 degrees to the Ulysses-Sun line. For channels with 8
sectors, sector 1 is the 45 degree sector which starts from
the sun-crossing line and extends in the sense of spacecraft
rotation. Sector 2 begins at the angle that sector 1 ends and
continues in the sense of spacecraft rotation, etc, continuing
through to sector 8 which ends at the the sun-crossing line.
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