5.1.1 Introduction

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)):

1. 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 SJT93.
2. 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.
3. 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.

1. 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.”
2. 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.
3. 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