The inferred spatial distribution, inferred dust grain properties, and abundances of inner source pickup ions have been discussed recently by Gloeckler et al. [this issue] and Schwadron et al. [this issue]. The abundances of all observed inner source constituents, with the exception of protons, appear to be very similar to solar wind abundances. This fact strongly constrains the production mechanism that appears to be predominantly caused by the absorption of solar wind ions by heliospheric grains and the subsequent reemission of neutrals. Inner source protons, however, appear depleted with an observed abundance of only 20% of the solar wind abundance. Here we point out that inner source hydrogen scatters Lyman α radiation, causing a radiation pressure comparable to gravity and leading to its effective loss. Under typical conditions in the fast solar wind we predict that this effect would lead to an abundance of inner source protons ∼ 55% of the solar wind abundance. We conclude that this process contributes to but probably does not entirely explain the observed depletion. The merits and drawbacks of other possible depletion mechanisms are also discussed.
