The Solar Wind Charge-Exchange Production Factor for Hydrogen

TL;DR

This paper determines the solar wind charge-exchange production factor for hydrogen in the 1/4 keV band using ROSAT data and simulations, revealing it is higher than previous estimates and highlighting its importance for astrophysical observations.

Contribution

The study provides a new, higher estimate of the charge-exchange production factor for hydrogen in the 1/4 keV band, derived from ROSAT data and magnetohydrodynamic simulations, improving understanding of soft X-ray background sources.

Findings

The production factor is $3.8 o4.3 imes 10^{-20}$ count degree$^{-2}$ cm$^4$.

Previous estimates underestimated the production factor by a factor of 1.8 to 4.5.

The method is not applicable to the 3/4 keV band due to its dependence on oxygen lines.

Abstract

The production factor, or broad band averaged cross-section, for solar wind charge-exchange with hydrogen producing emission in the ROSAT 1/4 keV (R12) band is $3.8\pm0.2\times10^{-20}$ count degree$^{-2}$ cm$^4$. This value is derived from a comparison of the Long-Term (background) Enhancements in the ROSAT All-Sky Survey with magnetohysdrodynamic simulations of the magnetosheath. This value is 1.8 to 4.5 times higher than values derived from limited atomic data, suggesting that those values may be missing a large number of faint lines. This production factor is important for deriving the exact amount of 1/4 keV band flux that is due to the Local Hot Bubble, for planning future observations in the 1/4 keV band, and for evaluating proposals for remote sensing of the magnetosheath. The same method cannot be applied to the 3/4 keV band as that band, being composed primarily of the oxygen lines, is far more sensitive to the detailed abundances and ionization balance in the solar wind. We also show, incidentally, that recent efforts to correlate XMM-Newton observing geometry with magnetosheath solar wind charge-exchange emission in the oxygen lines have been, quite literally, misguided. Simulations of the inner heliosphere show that broader efforts to correlate heliospheric solar wind charge-exchange with local solar wind parameters are unlikely to produce useful results.