  A science goal of many future X-ray observatories is mapping the cosmic web
through deep exposures of faint diffuse sources. Such observations require low
background and the best possible knowledge of the remaining unrejected
background. The dominant contribution to the background above 1-2 keV is from
Galactic Cosmic Ray protons. Their flux and spectrum are modulated by the solar
cycle but also by solar activity on shorter timescales. Understanding this
variability may prove crucial to reducing background uncertainty for ESA's
Athena X-ray Observatory and other missions with large collecting area. We
examine of the variability of the particle background as measured by ACIS on
the Chandra X-ray Observatory and compare that variability to that measured by
the Alpha Magnetic Spectrometer (AMS), a precision particle detector on the
ISS. We show that cosmic ray proton variability measured by AMS is well matched
to the ACIS background and can be used to estimate proton energies responsible
for the background. We discuss how this can inform future missions.
