Conclusions: • The distance to the Earth is the dominant parameter. As expected, the background is lower close to apogee, but ... • There is a marked asymmetry in the behaviour of the background when moving away from perigee and when approaching it • Generally speaking, the background level is higher at the end of the revolution than at the beginning • The high background time does not extend usually beyond 12 hours (phase < 0.25) after perigee • In some epochs of the year the background starts

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... to rise shortly after apogee (phase 0.65) • The behaviour of the background presents a pronounced seasonal effect • Near apogee, seasonal variations are not larger than a factor three • Near perigee, seasonal variations are as large as a factor eight • The seasonal variations are not exactly annual; the post-apogee radiation peak near Spring is occurring earlier every year.