The Characterization of Secondary Interstellar Neutral Oxygen beyond the Heliopause: A Detailed Analysis of the IBEX-Lo Oxygen Observations

Abstract

In this study, we analyze the directional distribution of the secondary interstellar neutral (ISN) O population observed by the IBEX-Lo neutral atom camera on the Interstellar Boundary EXplorer (IBEX) via the comparison with simulated ISN O intensity maps produced by an analytical model. In the analytical model, we assume that there are primary and secondary ISN populations at the heliopause. We further assume that each population is represented by a Maxwellian velocity distribution function with its own flow parameters. For the viewing directions of IBEX-Lo, we compute the incoming atom speeds at the heliopause with a Keplerian equation of motion in the solar gravity field. Then, we calculate analytically the distribution function to obtain the ISN intensities at Earth’s orbit. We compare the simulated O intensity maps with the IBEX-Lo O sky map to determine the most likely flow parameters of the secondary ISN O population. Using this method, we find the most likely flow parameters of the secondary ISN O population: Vₛₑ꜀ᵢₛₙₒ= 11 ± 2.2 km s⁻¹, λₛₑ꜀ᵢₛₙₒ = 67° ± 1 °. 5, βₛₑ꜀ᵢₛₙₒ = -12° ± 1 °. 6, and Tₛₑ꜀ᵢₛₙₒ = 10,000 ± 1500 K. The results indicate that the secondary ISN O flow direction is deflected toward lower ecliptic longitude and higher negative ecliptic latitude from the ISN gas flow direction at the heliopause. The secondary ISN O flow direction is more deflected from the ISN gas flow direction than the secondary ISN He flow direction.