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Interstellar neutral gas atoms penetrate the heliopause and reach 1~au, wherethey are detected by IBEX. The flow of neutral interstellar helium through theperturbed interstellar plasma in the outer heliosheath (OHS) results increation of the secondary population of interstellar He atoms, the so-calledWarm Breeze, due to charge exchange with perturbed ions. The secondarypopulation brings the imprint of the OHS conditions to the IBEX-Lo instrument.Based on a global simulation of the heliosphere with measurement-basedparameters and detailed kinetic simulation of the filtration of He in the OHS,we find the number density of interstellar He$^+$ population at $(8.98\pm0.12)\times 10^{-3}$~cm$^{-3}$. With this, we obtain the absolute density ofinterstellar H$^+$ $5.4\times 10^{-2}$~cm$^{-3}$ and electrons $6.3\times10^{-2}$~cm$^{-3}$, and ionization degrees of H 0.26 and He 0.37. The resultsagree with estimates of the Very Local Interstellar Matter parameters obtainedfrom fitting the observed spectra of diffuse interstellar EUV and soft X-Raybackground.

astrophysical journal/the astrophysical journal

Interstellar neutral helium in the heliosphere from IBEX observations.  VI. The He$^+$ density and the ionization state in the Very Local  Interstellar Matter