Ulysses HISCALE Pages
Document Reference: Dr. S. J. Tappin; Version 12 Oct., 1994
Once more the hydra of instrument backgrounds raises its hideous heads! Several issues
have emerged since the previous full assessment of the instrument backgrounds
(HISCALE BACKGROUNDS REVISITED (Revised Edition) (7 Oct. 93) (hereafter
referred to as SJT93)):
- In recent months some channels (most notably DE1) have been dropping consistently below
the SJT93 level, indicating that these channels had not reached background at the time of
- In other channels (DE4 being one of the most dramatic) there is a steady increase in the
background amounting to nearly a factor of 2.
- The attempts to understand this latter phenomenon have led to a fundamental questioning
of what precisely is background and what is a signal.
In this document I attempt to summarise the current state of our knowledge of the
instrument backgrounds. This is, however (as any analysis presented prior to the
completion of the mission must be), an interim description to be revised as new
information becomes available. The aim must therefore be to provide a prescription by
which backgrounds may be subtracted to a tolerable degree of accuracy and in a manner
appropriate to the investigation in progress.
Previous determinations: There have been several attempts to determine the
instrument background levels with varying scopes and degrees of completeness.
- The first attempt was in the autumn of 1991, Instrumental Backgrounds in HISCALE
by GMS (hereafter GMS91). This study selected 2 intervals early in the mission (one for
ions and the other for electrons) at which the counting rates appeared to reach their
lowest levels and to show signs of bottoming out.
- Some 2 years later SJT made a determination of the post-Jupiter levels; this is the
SJT93 statistical determination already mentioned above. For the pre-encounter period the
GMS91 values were retained.
- More recently GMS has made a closer analysis of the channels involved in the galactic
electrons study. This document, Background Analysis for the High-Latitude Cosmic Ray
Study (hereafter GMS94), showed that:
- Several of the SJT93 rates were too high.
- The time and/or latitude variation of the backgrounds was far from simple.
Continue: 5.1.2 Method
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Updated 1/22/09, T. Hunt-Ward