The Solar-Cycle Temporal Variation of the Solar Wind Charge Exchange
  X-ray Lines
  Solar wind charge exchange (SWCX) is the primary contamination to soft X-ray
emission lines from the Milky Way (MW) hot gas. We report a solar-cycle
($\approx 10$ yr) temporal variation of observed \ion{O}{7} and \ion{O}{8}
emission line measurements in the {\it XMM-Newton} archive, which is tightly
correlated with the solar cycle traced by the sunspot number (SSN). This
temporal variation is expected to be associated with the heliospheric SWCX.
Another observed correlation is that higher solar wind (SW) fluxes lead to
higher O VII or O VIII fluxes, which is due to the magnetospheric SWCX. We
construct an empirical model to reproduce the observed correlation between the
line measurements and the solar activity (i.e., the SW flux and the SSN). With
this model we discovered a lag of $0.91_{-0.22}^{+0.20}$ yr between the O VII
flux and the SSN. This time lag is a combination of the SW transit time within
the heliosphere, the lag of the neutral gas distribution responding to solar
activity, and the intrinsic lag between the SSN and the launch of a high-energy
SW (i.e., $\rm O^{7+}$ and $\rm O^{8+}$). MW O VII and O VIII fluxes have mean
values of 5.4 L.U. and 1.7 L.U., which are reduced by $50\%$ and $30\%$,
compared to studies where the SWCX contamination is not removed. This
correction also changes the determination of the density distribution and the
temperature profile of the MW hot gas.
