The Wide Field Imager (WFI) is one of two focal plane detector systems of ESA's Advanced Telescope for High ENergy Astrophysics (ATHENA) X-ray observatory. The Science Products Module (SPM) will have on-board processing algorithms that will reduce the ATHENA WFI particle background level significantly by improving background rejection on board and in post-processing on the ground. To this end, we examine the full frame observations from existing X-ray telescopes to understand and characterize the physics of the particle background. In particular, we determine phenomenological correlations between high energy particle events and X-ray events to improve the rejection of particle background events. We will present our results from the Swift XRT and XMM-Newton PN full frame data analysis in this talk. We will also discuss how these results could be used to reduce the expected background in the ATHENA WFI observations by the SPM processing.