Soft Proton Fluxes in and Around the Earth's Magnetotail

The second L-class (large) mission in European Space Agency's Cosmic Vision program will be an X-ray telescope named Athena, planned to operate at the L2 Lagrange point of the Sun-Earth system. Current large X-ray space telescopes like XMM-Newton and Chandra have encountered periods of unexpectedly high background due to protons in the energy range from 10 keV to 1 MeV (called soft protons hereafter). This is an important issue for Athena, as no X-ray telescope has been deployed at L2 so far and the soft proton environment there is poorly known. We analyze data from the Artemis and Advanced Composition Explorer spacecraft and find that fluxes of both solar and geomagnetospheric origin significantly contribute to the soft proton populations in earth's magnetotail. We then estimate the background from the measured fluxes in Athena's X-ray instruments and compare it with the science requirement. Our findings reinforce the argument for developing a means to suppress the soft proton flux before it reaches the detectors such as with a magnetic diverter.