Observations of interstellar neon in the helium focusing cone

Extended pickup ion distributions in the heliosphere have been found to originate either from interstellar neutral gas or from the so-called inner source. Because of the motion of the Sun through the interstellar medium, interstellar neutral gas flows through the inner heliosphere and for some of the species is focused on the downwind side by the gravitation of the Sun. This gravitational focusing cone, which is absent for inner source ions, has been identified for He with UV backscattering and pickup ion observations and is used to diagnose the interstellar parameters. Using the large geometric factor of the Plasma and Suprathermal Ion Composition instrument on the Solar Terrestrial Relations Observatory mission, we have studied heavy pickup ions in the mass-per-charge range 12-20 during 2007 and 2008. We have clearly identified C⁺, N⁺, O⁺, the H₂O⁺ group, and Ne⁺ among the heavy pickup ions. Out of these, Ne⁺ shows clear enhancements during two consecutive focusing cone passages, as evidenced by concurrent He⁺ enhancements during these periods, whereas all the other species are evenly distributed. This is the first observation of the interstellar focusing cone for Ne, whose survival probability as neutrals is only slightly lower than that of He and substantially higher than for other elements.