Time variability of the geocoronal solar-wind charge exchange in the direction of the celestial equator

We report on the detection of a time variable O VII line emission in a deep 100ks Suzaku X-ray Imaging Spectrometer spectrum of the galactic ridge X-ray emission. The observed line intensity is too strong (11 ±2 line unit or photon cm^-2 s^-1 sr^-1) to be emitted inside the heavily obscured galactic disk. It showed a factor of two time variation that showed a significant (∼4σ) correlation with the solar-wind O ⁷⁺ ion flux. The high line intensity and good time correlation with the solar wind strongly suggests that it originated from geocoronal solar-wind charge-exchange emission. We discuss the X-ray line intensity while considering a line-of-sight direction and also theoretical distribution models of the neutral hydrogen and solar wind around Earth. Our results indicate that X-ray observations of geocoronal solar-wind charge-exchange emission can be used to constrain these models.