| Abstract
| - In this Letter we report on our study of the filtration of interstellar atoms of hydrogen, oxygen and nitrogen in the interaction region between the solar wind and the local interstellar medium. The filtration has great importance for the determination of local interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup ions by Ulysses and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtration of the different elements depends on the level of their coupling with the plasma in the interaction region. We study the dependence of the filtration on the local interstellar proton and H atom number densities and evaluate the effects of charge exchange and electron impact ionization on the filtration. We explore the influence of electron temperature in the inner heliosheath on the filtration process. Using our filtration coefficients and SWICS/Ulysses pickup ion measurements we conclude $n_{\rm OI,LIC}=(7.8 \pm 1.3)\times 10^{-5}$ cm -3 and $n_{\rm NI,LIC}=(1.1 \pm 0.2)\times 10^{-5}$ cm -3.
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