Nature http://dx.doi.org/10.1038/nature13525 (2014)
The origin of a million-degree bubble of ionized gas surrounding the Sun is hotly debated. Is the gas produced within the Solar System, or is it of interstellar origin? Such details are important for models of star and galaxy formation, for example. Massimiliano Galeazzi and colleagues have used data from the Diffuse X-ray Emission from the Local Galaxy (DXL) space mission to quantify how much X-ray emission is locally sourced.
A volume of neutral hydrogen (and about 15% helium) gas within a small interstellar cloud intersecting the Solar System can become ionized through the interaction with the charged solar wind, in a process known as charge exchange. When an electron returns to the ground state of an ion, soft X-rays are emitted. This type of gas forms a cone shape downstream from the Sun. Interstellar gas is not similarly focussed.
Analysis of the DXL data shows that roughly 40% of the soft X-ray flux is produced by charge exchange. This means that most of the flux is from hot interstellar plasmas rather than from the solar wind.
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Chiao, M. Local hot bubble. Nature Phys 10, 547 (2014). https://doi.org/10.1038/nphys3066