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Interest in astrophysical sources of charge exchange X-rays has been growing steadily since the discovery of X-ray emission from the comet Hyakutake with ROSAT in 1996. Since then, charge exchange has been observed between solar wind ions and neutrals in the geocorona and in the atmospheres of Mars and Jupiter. Charge exchange with interstellar neutrals within the heliosphere between solar wind ions and neutral hydrogen and helium from the interstellar medium is now acknowledged as contributing a considerable (although currently unknown) fraction of the soft X-ray background. We make a brief survey of the heliospheric, Galactic, and extragalactic systems in which charge exchange has been observed or is predicted to take place. Experiments measuring velocity dependent cross-section and line ratios for Lyman-series lines and He-like triplets are needed to check current theoretical models of charge exchange emission and aid interpretation of observations. We point out a number of systems that are of astrophy...

International audience; Charge exchange (CX) is a basic physical process involving the transfer of bound electrons between colliding particles. In the solar system, highly-charged solar wind ions collide with neutral particles from cometary and planetary (e.g. Mars, Earth) environments, as well as interstellar atoms sweeping through the heliosphere, and produce X-rays through radiative decay. I will present an overview of our current knowledge of the CX X-ray emission, focusing mainly on the heliospheric component which, along with CX emission from the Earth's geocorona, surrounds Earth-bound observatories and adds a variable background to all X-ray observations. Current studies combine theoretical modeling and observational analyses from Suzaku, XMM-Newton and Chandra in order to constrain this emission and its contribution to the diffuse soft X-ray background.

The soft X‐ray background observed from Earth contains contributions not only from outside the solar system such as the local bubble but also contributions from within the solar system, including contributions from the interplanetary medium, from the terrestrial geocorona, and from the magnetosheath. Great effort was spent on removing non‐cosmic contamination from data collected during the ROSAT all‐sky survey. Some of the contamination is the X‐ray emission produced from charge exchange of solar wind ions with interstellar and geocoronal neutral gas. The time‐varying component of this contamination was removed for the ROSAT survey, but the steady‐state component was not. In this chapter, we present ROSAT 1/4‐keV and 3/4‐keV band all‐sky maps of the cosmic soft X‐ray emission with the steady state heliospheric and geocoronal components removed via modeling procedures. These new determinations of the “true” cosmic background X‐ray emission will allow a re‐interpretation of the nature of the local hot bubble. In particular, the thermal pressure of the bubble gas must be about a factor of two less than the pressures deduced from the original ROSAT all‐sky survey.

The Chandra Deep Field‐South (CDF‐S) observing program, comprising 2 Ms of data collected near Solar Maximum (1999 and 2000) and during Solar Minimum (2007), provides a uniquely rich and informative data set for investigation of solar wind charge exchange (SWCX) and the soft X‐ray background (SXRB). When combined with data on solar wind composition from the ACE satellite, these observations permit studies of the effects of solar wind stratification (fast versus slow wind) and viewing geometry (through the heliosphere and geocorona) on SWCX emission. We discuss preliminary results from our analysis of these data, including an upper limit on the true cosmic (non‐SWCX) SXRB in the 3/4‐keV band in the direction of the CDF‐S.

The X‐ray Imaging Spectrometer (XIS) onboard Suzaku has an energy resolution sufficient to detect O emission lines from the hot plasma of T>106 K. Suzaku observations will produce new understaning of the physical condition of the hot phase of the Universe. New trials to quantitatively evaluate the Solar Wind Charge Exchange (SWCX) emission in the geocorona and the heliospheric region, the “M band problem” of the soft X‐ray diffuse emission and the abundance pattern in the halo gas around galaxies are presented.

We consider uses of observatories on the Moon for addressing problems in composition and structure of planetary atmospheres. The focus is on the science which can be uniquely done from the Moon or, at least, best done from the Moon. These fall into three categories, phenomena close to the solar direction, phenomena requiring removal from the near‐Earth environment for observation (e.g., phenomena requiring continuous time coverage over time scales of hours, observations required outside the magnetosphere, and observations required away from the geocorona), and phenomena not observable from the Earth’s surface which require very large telescopes or interferometric arrays.