The radiation background will be variable around the XMM operational orbits, depending on the satellite's location with respect to Earth's magnetosphere. For useful XMM science observations to be conducted, the minimum satellite elevation is 40,000 km. Closer to Earth, the background is prohibitively high. Because of this, about 40 hours of the 48-hour orbital period can be used for science observations, with a total target visibility period of approximately 145 ks per orbit. Thus, due to the highly elliptical orbit, XMM spends most of its time outside Earth's radiation belts. Particle background outside the belts is dominated by the solar particle emission. The incident flux is therefore coupled to the level of solar activity. During intense solar flares the XMM payload instruments may be switched off.