Neutral interstellar hydrogen in the inner heliosphere under the influence of wavelength-dependent solar radiation pressure

Abstract

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 ∼ 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 ∼ 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. © 2008 ESO.