Solar system X-rays from charge exchange processes

Abstract

While X-ray astronomy began in 1962 and has made fast progress since then in expanding our knowledge about where in the Universe X-rays are generated by which processes, it took one generation before the importance of a fundamentally different process was recognized. This happened in our immediate neighborhood, when in 1996 comets were discovered as a new class of X-ray sources, directing our attention to charge exchange reactions. Charge exchange is fundamentally different from other processes which lead to the generation of X-rays, because the X-rays are not produced by hot electrons, but by ions picking up electrons from cold gas. Thus it opens up a new window, making it possible to detect cool gas in X-rays (like in comets), while all the other processes require extremely high temperatures or otherwise extreme conditions. After having been overlooked for a long time, the astrophysical importance of charge exchange for the generation of X-rays is now receiving increased general attention. In our solar system, charge exchange induced X-rays have now been established to originate in comets, in all the planets from Venus to Jupiter, and even in the heliosphere itself. In addition to that, evidence for this X-ray emission mechanism has been found at various locations across the Universe. Here we summarize the current knowledge about solar system X-rays resulting from charge exchange processes.