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

Abstract

We study the influence of the non-flat shape of the solar Lyman-alpha line on the distribution of neutral interstellar hydrogen in the inner heliosphere and assess importance of this effect for interpretation of heliospheric in situ measurements. Based on available data, construct a model of evolution of the solar Lyman-alpha line profile with solar activity. Modify an existing test-particle code calculating distribution of neutral interstellar hydrogen in the inner heliosphere to take into account the dependence of radiation pressure on radial velocity. Discrepancies between the classical and Doppler models appear at ~ 5 AU and increase towards the Sun from a few percent to a factor of 1.5 at 1 AU. The classical model overestimates density everywhere except a ~ 60 deg cone around the downwind direction, where a density deficit appears. The magnitude of discrepancies depends appreciably on the phase of 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, as 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 and degree, which, when unaccounted for, would bring an error of a few degrees and a few km/s in determination of the bulk velocity vector at the termination shock. The Doppler correction is appreciable for in situ observations of neutral H populations and their derivatives performed a few AU from the Sun.