Interstellar Neutral Helium in the Heliosphere from IBEX Observations. VI. The He⁺ Density and the Ionization State in the Very Local Interstellar Matter

Article

Bzowski, M., Czechowski, A., Frisch, P. C., Fuselier, S. A., Galli, A., Grygorczuk, J., Heerikhuisen, J., Kubiak, M. A., Kucharek, H., McComas, D. J., Möbius, E., Schwadron, N. A., Slavin, Jonathan, Sokół, J. M., Swaczyna, P., Wurz, P., and Zirnstein, E. J. 2019. "Interstellar Neutral Helium in the Heliosphere from IBEX Observations. VI. The He Density and the Ionization State in the Very Local Interstellar Matter." The Astrophysical Journal, 882 60. https://doi.org/10.3847/1538-4357/ab3462.

Interstellar neutral gas atoms penetrate the heliopause and reach 1 au, where they are detected by Interstellar Boundary Explorer (IBEX). The flow of neutral interstellar helium through the perturbed interstellar plasma in the outer heliosheath (OHS) results in the creation of a secondary population of interstellar He atoms, the so-called Warm Breeze, due to charge exchange with perturbed ions. The secondary population brings the imprint of the OHS conditions to the IBEX-Lo instrument. Based on a global simulation of the heliosphere with measurement-based parameters and detailed kinetic simulation of the filtration of He in the OHS, we find the number density of the interstellar He⁺ population to be (8.98 ± 0.12) × 10^-3 cm^-3. With this, we obtain the absolute density of interstellar H⁺ as 5.4 × 10^-2 cm^-3 and that of electrons as 6.3 × 10^-2 cm^-3, with ionization degrees of 0.26 for H and 0.37 for He. The results agree with estimates of the parameters of the Very Local Interstellar Matter obtained from fitting the observed spectra of diffuse interstellar EUV and the soft X-ray background.