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Four different components have been identified in the non-cosmic background of PSPC observations. The two best understood components are the particle background and the background caused by solar X-rays scattered into the field of view by Earth's residual atmosphere along the line of sight. The other components have been identified through their temporal variations, as observed in the ROSAT sky survey [Snowden and Schmitt1990, Voges1992, Snowden et al.1995a].
One consists of short term enhancements (STEs)
which include auroral X-rays, the other of long term
enhancements (LTEs) which are of unknown origin.
STEs have a variation time scale between much less than 1 orbit
(1 orbit 
95 minutes), and 
orbits.
LTEs have durations of 1-2 days or more but were probably
present at some level during more than 80% of the sky survey
(and so probably affect most pointed observations,
at least in the early part of the mission, as well).
Of these various contributors to the non-cosmic background, only the particle background is completely intrinsic to the PSPC and satellite structure. The scattered solar X-ray background and in all probability the LTEs originate externally to ROSAT as a diffuse X-ray flux and are impossible to discriminate against in event processing. STEs are likely to have a variety of sources from auroral X-rays to enhanced particle precipitation producing X-rays from the mirror surfaces. Discrimination against these X-rays in event processing is also impossible.