Solar Wind Charge Exchange

Charge exchange emission can occur when ions interact with neutral atoms or molecules; one or more electrons are transferred to the ion into an excited state. In the subsequent relaxation of the ion (with the charge now reduced by the number of electrons captured), a cascade of photons may be emitted. Charge exchange is increasingly being seen as an important emission process throughout the Universe, such as in supernova remnants, starburst galaxies (e.g. M82) and between colliding stellar winds.
The charge exchange process. Credit: D. Bodewits (University of Maryland)

Charge exchange results in line emission at energies characteristic of the ions involved. When the ions originate in the solar wind this process is known as solar wind charge exchange (SWCX). The ions that result in the emission of X-rays include highly-charged oxygen, carbon and neon, which are all present in low percentages in the solar wind. SWCX emission can be seen throughout the Solar System, for example in planetary exospheres such as the Earth, Mars and Jupiter and also in the coma of comets. The emitted X-rays can be used to identify compositional and flux changes in the solar wind and, especially in the case of cometary X-ray emission, used to probe the state of the solar wind at different heliospheric latitudes.

SWCX emission could be used to image the Earth's magnetosheath. The magnetosheath is the area near the Earth where the magnetic field, embedded in the solar wind plasma, is confronted by the Earth's magnetic field. This results in the magnetosphere; a cavity of Earth-confined plasma which protects the Earth from the harsh conditions within the solar wind. This picture, however, is not static as the Earth system responds quickly to changes in the solar wind density and velocity. Turbulent processes and magnetic reconnection in the magnetosheath result in phenomena such as the Aurora Polaris (Borealis or Australis). More violent phenomena, such as Coronal Mass Ejections, (clouds of very fast moving plasma moving through the Solar System), have been observed using XMM-Newton, see the spectrum below. At the University of Leicester we quantify the level of SWCX seen by X-ray observatories such as XMM-Newton, and study what can be learnt about the solar wind through the observed emission.

X-ray spectrum
Spectrum resulting from a Coronal Mass Ejection passing by the Earth, as seen in XMM-Newton data

We are also involved in the design of the AXIOM mission concept. This mission, if funded, will use wide-field X-ray optics to image areas of the Earth's magnetosheath. Using spacecraft such as AXIOM we wish to answer questions regarding the shape and response of the magnetosheath to changes in the solar wind, and how solar wind plasma is funnelled into the magnetosphere. As a society, we are becoming more and more reliant on space-based technology, much of which is found in geostationary orbits. Geostationary orbits are particular vulnerable to space weather, as their distance from Earth means that satellites at these orbits can be exposed to, and potentially seriously damaged by, the solar wind as it pushes up against the Earth's magnetic field.

The AXIOM instrument concept, which would use SWCX X-ray emission to study the Earth's magnetosheath

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