Evidence for Solar-Wind Charge-Exchange X-Ray Emission from the Earth's Magnetosheath

We report an apparent detection of the C VI 4p to 1s transition line at 459eV, during a long-term enhancement (LTE) in the Suzaku north ecliptic pole(NEP) observation of 2005 September 2. The observed intensity of the line iscomparable to that of the C VI 2p to 1s line at 367 eV. This is strong evidencefor the charge-exchange process. In addition to the C VI lines, emission linesfrom O VII, O VIII, Ne X, and Mg XI lines showed clear enhancements. There arealso features in the 750 to 900 eV range that could be due to some combinationof Fe XVII and XVIII L-lines, higher order transitions of O VIII (3p to 1s and6p to 1s), and a Ne IX line. From the correlation of the X-ray intensity withsolar-wind flux on time scales of about half a day, and from the short-term(~10 minutes) variations of the X-ray intensity, these lines most likely arisefrom solar-wind heavy ions interacting with neutral material in the Earth'smagnetosheath. A hard power-law component is also necessary to explain the LTEspectrum. The origin of this component is not yet known. Our results indicatethat solar activity can significantly contaminate Suzaku cosmic X-ray spectrabelow ~1 keV. Recommendations are provided for recognizing such contaminationin observations of extended sources.Comment: 8 pages, 8 figures, accepted by Publications of the Astronomical Society of Japa