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

We report an apparent detection of the CVI 4p to 1s transition line at 459 eV, during a long-term enhancement (LTE) in the Suzaku north ecliptic pole (NEP) observation of 2005 September 2. The observed intensity of the line is comparable to that of the CVI 2p to 1s line at 367 eV. This is strong evidence for the charge-exchange process. In addition to the CVI lines, emission lines from OVII, OVIII, NeX, and MgXI lines showed clear enhancements. There are also features in the 750 to 900 eV range that could be due to some combination of FeXVII andXVIII L-lines, higher order transitions of OVIII (3p to 1s and 6p to 1s), and a Ne IX line. From the correlation of the X-ray intensity with solar-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 arise from solar-wind heavy ions interacting with neutral material in the Earth’s magnetosheath. A hard power-law component is also necessary to explain the LTE spectrum. The origin of this component is not yet known. Our results indicate that solar activity can significantly contaminate Suzaku cosmic X-ray spectra below ∼ 1 keV. Recommendations are provided for recognizing such contamination in observations of extended sources.