Earth’s Exospheric X-ray Emissions

In this chapter we explore solar wind charge exchange X-ray emission emitted from the Earth’s exosphere, following an interaction between heavy ions in the Sun’s solar wind and the cloud of neutral hydrogen that surrounds the Earth. We describe the spatial, temporal, and spectral characteristics of this emission, as deduced from several astronomical X-ray missions. X-ray emissivities are commonly estimated using a magnetohydrodynamic model of the Earth’s magnetosphere and by making assumptions regarding the heavy ion composition of the solar wind and the exospheric hydrogen density. The estimated emissivities may then be integrated along a particular line of sight, dependent on the operational or theoretical X-ray astronomical telescope of interest. We stress how knowing the distribution of exospheric neutral hydrogen is key to understanding the component contributions to the X-ray emission. A careful consideration of charge exchange X-ray emission is important for X-ray astronomers wishing to account for all sources of background or foreground emission that must be eliminated when analyzing their science target of interest. This is particularly important for those astronomers studying diffuse and extended sources. Far from being entirely problematic however, this emission may be employed to image large areas of near-Earth space and in doing so probe outstanding questions regarding magnetospheric-ionospheric couplingMagnetospheric-ionospheric coupling. For example, wide-field images may be used to determine the location of the magnetopause boundary as it responds to the incoming solar wind and interplanetary magnetic field. We provide a brief overview of some missions, currently in development, that exploit exospheric charge exchange emission.