Long-Term Variability of the O VII Line Intensity toward the Lockman Hole Observed with Suzaku from 2006 to 2011

Long-term time variabilities of the O VII (0.57 keV) emission in the soft X-ray diffuse background were studied using six Suzaku annual observations of the blank sky towards the Lockman Hole made from 2006 to 2011. After time intervals in which the emission was enhanced on time scales of a few tens of ks were removed, the O VII intensity was found to be constant from 2006 to 2009 within 90% statistical errors. The intensity in 2010 and 2011 was higher by 2-3 LU (= photons s(-1) cm(-2) sr(-1)) than the earlier values. The most plausible origin of the fast variable component is Solar wind charge exchange (SWCX). The intensity increase is not positively correlated with the proton flux at the L1 point. Since all of the observations were made in the same season of a year, the variation cannot be explained by parallax of the SWCX-induced X-ray emission from the Heliosphere. We consider that it is related to geometrical changes of slow and fast solar wind structures associated with the L1 year solar activity. The observed variation was compared with that expected from the SWCX-induced X-ray emission model.