X-ray emission from charge-exchange: An astrophysical plasma diagnostic tool.

Astrophysical x-rays typically come from hot collisional plasmas, such as the solar corona or supernova remnants. However, x-rays can also be produced in cooler gas by charge exchange (CX) collisions between neutrals and highly-charged ions. The CX mechanism applied to the solar wind has been shown to generate x-ray emission at comets, in the terrestrial magnetosheath, and throughout the heliosphere (where the solar wind interacts with incoming interstellar neutral gas). Heliospheric emission is thought to make a significant contribution to the observed soft x-ray background (SXRB). Efforts are underway to distinguish this contribution from emission due to hot interstellar gas and the galactic halo. X-rays from CX could provide diagnostic information (e.g., line ratios) on regions where hot astrophysical plasma comes into contact with neutral gas.