Fortunately the closed position of the filter wheel places 1.05 mm of aluminium in the focused beam and completely stops the protons. For all perigee passages the filter wheel is rotated to the closed position before the spacecraft encounters the outer radiation belts. This has prevented damage by soft protons in the belts. Soft protons are also found outside the belts, probably accelerated in the magnetopause. When the flux of these is high the filter wheel is rotated to the closed position to protect the CCDs.
Lower fluxes can contribute to the background in the image during observations.
It is not known if the prevalence of these clouds of soft protons will change as solar activity decreases, or as the orbit of XMM-Newton evolves. On average several hours per orbit have a high soft proton background, or are lost completely because the filter wheel has to be closed. Figure 11 shows an image collected while the soft proton flux was relatively high compared with a low proton flux image. There have been two large solar flares so far since the launch of XMM-Newton . After the first flare a measurable degradation of the CTE was observed, equivalent to several months' passages through the radiation belts.