Aims. By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements near 5 AU close to the ecliptic right after the previous solar minimum, this paper presents a determination of the neutral interstellar H density at the solar wind termination shock and implications for the density and ionization degree of hydrogen in the local interstellar cloud. Methods. The density of neutral interstellar hydrogen at the termination shock was determined from the local pickup ion production rate as obtained close to the cut-off in the distribution function at aphelion of Ulysses. As shown in an analytical treatment for the upwind axis and through kinetic modeling of the pickup ion production rate at the observer location, with variations in the ionization rate, radiation pressure, and the modeling of the particle behavior, this analysis turns out to be very robust against uncertainties in these parameters and the modeling.Results. Analysis using current heliospheric parameters yields the H density at the termination shock equal to 0.087 ± 0.022 cm −3 , including observational and modeling uncertainties.

We discuss a consolidation of determinations of the density of neutral interstellar H at the nose of the termination shock carried out with the use of various data sets, techniques, and modeling approaches. In particular, we focus on the determination of this density based on observations of H pickup ions on Ulysses during its aphelion passage through the ecliptic plane. We discuss in greater detail a novel method of determination of the density from these measurements and review the results from its application to actual data. The H density at TS derived from this analysis is equal to 0.087 ± 0.022 cm −3 , and when all relevant determinations are taken into account, the consolidated density is obtained at 0.09 ± 0.022 cm −3 . The density of H in CHISM based on literature values of filtration factor is then calculated at 0.16 ± 0.04 cm −3 .

Context. With the plethora of detailed results from heliospheric missions such as Ulysses and SOHO and at the advent of the first mission dedicated to in situ studies of neutral heliospheric atoms IBEX, we have entered the era of precision heliospheric studies. Interpretation of these data requires precision modeling, with second-order effects quantitatively taken into account. Aims. We study the influence of the non-flat shape of the solar Lyman-α line on the distribution of neutral interstellar hydrogen in the inner heliosphere and assess the importance of this effect for interpretation of heliospheric in situ measurements. Methods. Based on available data, we (i) constructed a model of evolution for the solar Lyman-α line profile with solar activity; (ii) modified an existing test-particle code used to calculate the distribution of neutral interstellar hydrogen in the inner heliosphere so that it takes the dependence of radiation pressure on radial velocity into account; and (iii) compared results of the old and new version. Results. Discrepancies between the classical and Doppler models appear between ∼5 and 3 AU and increase towards the Sun from a few percent to a factor of 1.5 at 1 AU. The classical model overestimates the density everywhere except for a ∼60• cone around the downwind direction, where a density deficit appears. The magnitude of the discrepancies appreciably depends on the phase of the solar cycle, but only weakly on the parameters of the gas at the termination shock. For in situ measurements of neutral atoms performed at ∼1 AU, like those planned for IBEX, the Doppler correction will need to be taken into account, because the modifications include both the magnitude and direction of the local flux by a few km s −1 and degrees, respectively, which, when unaccounted for, would introduce an error of a few km s −1 and degrees in determination of the magnitude and direction of the bulk velocity vector at the termination shock. Conclusions. The Doppler correction is appreciable for in situ observations of neutral H populations and their derivatives performed a few AU from the Sun.

Context. We study the feasibility of detection of neutral interstellar deuterium by the forthcoming NASA SMEX mission IBEX. Aims. Using numerical simulations, we study the absolute density and flux in Earth orbit of neutral interstellar deuterium and check its detectability using IBEX. Methods. Our simulations were performed using the Warsaw 3D time-dependent test-particle model of neutral interstellar gas in the inner heliosphere, which was specially adapted to the case of deuterium, and state-of-the-art models of the ionization field and radiation pressure. The modeling predicted the density, bulk velocity, and flux of interstellar D at different positions of the Earth during the solar cycle. We paid particular attention to the time interval in which IBEX observations will be performed. Results. Using our simulations, we predict a large enhancement of deuterium abundance in Earth orbit with respect to the abundance at the termination shock. The energy of the D atoms at IBEX will be within the energetic sensitivity band of its Lo instrument, apart from during a short time interval between September and November each year. Because of the specific observing geometry of IBEX, there will be one opportunity each year to search for I/S D, when Earth is close to ecliptic longitude 165• , i.e. in March. Assuming that the TS abundance of D is identical as in the Local Cloud, which is equal to 1.56 × 10 −5 , and that the density of H at TS is 0.11 cm −2 s −1 , we estimate the expected relative flux to be approximately 0.015 cm −2 s −1 , which corresponds to the local absolute flux of about 0.007 cm −2 s −1 . The dependence of the expected flux on the phase of solar cycle is relatively weak. The flux scales proportionally to the density of deuterium at the termination shock and depends only weakly on the bulk velocity and temperature of the gas in this region.

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Dynamics of H, D, and heavy Energetic Neutral Atoms (ENA) between the termination shock and 1 AU is discussed in the context of the forthcoming NASA SMEX mission IBEX. In particular, effects of the velocity-dependent radiation pressure on atomic trajectories are considered and ionization losses between TS and 1 AU are studied. It is shown, among others, that most of the dynamical effects and ionization losses are induced within a few AU from the Sun, which translates to the time domain into $\sim 1 - 3$ solar rotations before detection. This loosens considerably time requirements for tracking the ionization and radiation pressure history to just prior 3 months. ENA seem excellent tracers of the processes within the heliospheric interface, with the transport effects between the termination shock and detector relatively mild and easy to account for.Comment: submitted to Proceedings of the 5-th IGPP Astrophysics Conference, Honolulu HI, March 2006; 6 page