Solar Wind Charge Exchange

The solar wind originates in the million-degree solar corona and flows out from the Sun at 300 to more than 800 km/sec (roughly one to two million miles per hour). Most of the ions in the wind are hydrogen and helium, but a small fraction are heavier elements such as carbon, oxygen, and neon. When those heavy ions, which have lost most or all of their electrons, collide with neutral gas in comets, planetary atmospheres, or very tenuous gas throughout the solar system, they emit X-rays via a process called charge exchange.

Solar wind charge exchange (SWCX) emission has been observed from several planets and dozens of comets. There is also a diffuse X-ray glow from the Earth's outer atmosphere and from the entire solar system as the solar wind streams out to the edges of the heliosphere, well beyond Pluto. Wherever our X-ray telescopes point, they look through this geocoronal and heliospheric glow, which contributes a large fraction (depending on energy and direction) of the Soft X-Ray Background (SXRB). Scientists at CfA and elsewhere observe this emission with telescopes such as Chandra, XMM, and Suzaku, model it using simulations of the solar wind's interaction with the Earth's magnetosphere, and study charge exchange spectra in laboratory experiments.