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

TL;DR

This paper investigates the pressure and ionization balance issues between the warm Circum-heliospheric Interstellar Medium and the hot Local Bubble, proposing that previous measurements were affected by foreground contamination and revising the understanding of ionization sources.

Contribution

It identifies and corrects for foreground contamination in X-ray background measurements, leading to revised estimates of ionization and pressure balances in the local interstellar environment.

Findings

Foreground contamination inflated previous electron density estimates.

Revised ionization rates suggest different sources influence the CHISM.

Pressure mismatch issues are partially resolved with new data interpretations.

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.