Suzaku observation of strong solar-wind charge-exchange emission from the terrestrial exosphere during a geomagnetic storm

We present an analysis of X-ray data exhibiting strong solar-wind charge-exchange emission obtained with the Suzaku satellite during a geomagnetic storm that occurred on 2005 August 31. A temporal variation of diffuse soft X-ray emission, correlating with the solar-wind proton flux, was found. The diffuse emission consisted of exospheric solar-wind charge exchange (geocoronal SWCX). We extracted the variable component of the spectrum, which is dominated by a sum of C V, CVI, NVI, NVII, OVII, and OVIII emission lines, predicted by the theoretical SWCX model. An analysis of any time correlation between the solar wind and the OVII line flux was conducted using the solar-wind data taken with the ACE and WIND satellites. We found that the observed SWCX intensity was 4-10 times higher than that calculated using a model of exospheric hydrogen density, and the O7+ flux observed with ACE at the L1 point. This suggests that the exospheric hydrogen density can be higher than that predicted by the hydrogen model, and/or that additional O7+ ions to the measured ACE fluxes could exist in the magnetosphere. Comparing this observation to past incidences of SWCX emission, as recorded by Suzaku, it was found that the SWCX intensity in this observation was strong, in spite of the fact that the line-of-sight direction did not traverse the sub-solar magnetosheath nor the magnetospheric cusps, where higher SWCX emission would be expected.