Solar system X-rays from charge exchange processes

• Published in: Astronomische Nachrichten 2012-04, Vol.333 (4), p.324-334
• Main Authors: Dennerl, K., Lisse, C.M., Bhardwaj, A., Christian, D.J., Wolk, S.J., Bodewits, D., Zurbuchen, T.H., Combi, M., Lepri, S.
• Format: Article
• Language: English
• Subjects: atomic processes; comets: general; planets and satellites: general; radiation mechanism: non-thermal; solar wind; X-rays: general
• ISSN: 0004-6337; 1521-3994
• DOI: 10.1002/asna.201211663

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 (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)