Due to its eccentric polar orbit with an apogee around , the COS-B satellite was exposed to the full cosmic-ray flux, unshielded by the earth's magnetic field which is modulated by the effects of solar activity on timescales of minutes to years, primarily in correspondence to the sun's 11-year cycle.
This orbit was chosen for technical advantages in data transmission to obtain long uninterrupted observation intervals (32 hours out of the 36 hour orbital period) and to gain observation time which in a low orbit for a spinning satellite is lost by earth occultation of the field of view.
On-board scintillation counters combined into the scaler-3 rate of the trigger telescope could be demonstrated to closely trace the cosmic-ray flux modulated by solar activity. When all gamma-ray data were available from the mission, the variable fraction of the COS-B gamma-ray background could be related to the scaler-3 rate. Unfortunately there remains the larger fraction of the likely instrumental background not modulated in time. A large modulation is expected only for low energy protons and especially electrons, which might be of special importance here, while for highly relativistic protons only a modulation of a few percent is occurring. Therefore a significant time invariant instrumental background is seen which remains indistinguishable from any possible celestial (galactic or extragalactic) isotropic emission.
The instrumental background, when described in the form of a sky photon intensity, need not necessarily appear isotropic and actually is found to show a variation with inclination to the telescope's axis.