Pressure and Ionization Balances in the Circum-Heliospheric Interstellar Medium and the Local Bubble

Abstract

A disconcerting mismatch of thermal pressures for two media in contact with each other, (1) the warm, Circum-heliospheric Interstellar Medium (CHISM) and (2) the very hot material within a much larger region called the Local Bubble (LB), has troubled astronomers for over two decades. A possible resolution of this problem, at least in part, now seems possible. We now understand that earlier estimates for the average electron density in the very hot LB plasma were inflated by an unrecognized foreground contamination to the low energy diffuse X-ray background measurements. This foreground illumination arises from photons emitted by charge exchange reactions between solar wind ions and neutral atoms from the interstellar medium that enter into the heliosphere. However, with the resolution of this problem comes a new one. The high ionization fraction of helium in the CHISM, relative to that of hydrogen, could be understood in terms of the effects from a strong flux of EUV and X-ray radiation coming from both the Local Bubble and a conductive interface around the CHISM. A revision of this interpretation may now be in order, now that the photoionization rate from radiation emitted by hot gas the Local Bubble is lower than previously assumed.

12 pages, 0 figures, to appear in Space Science Reviews, electronic version of journal article available at http://www.springerlink.com/content/l7015x3115k70663/?p=c9685269591242929cb06a370e36520b

References in corpus (5)

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• The search for the origin of the Local Bubble redivivus
• The Orientation of the Local Interstellar Magnetic Field
• Dependence of Heliospheric Lyman-alpha Absorption on the Interstellar Magnetic Field
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• The Solar Wind Charge-Exchange Production Factor for Hydrogen
• XMM-Newton and Suzaku X-ray Shadowing Measurements of the Solar Wind Charge Exchange, Local Bubble, and Galactic Halo Emission
• Scientific Objectives of the Hot Universe Baryon Surveyor (HUBS) Mission
• Are the heliosphere, very local interstellar medium, and local cavity in pressure balance with Galactic gravity