IBEX Data Release 9: Exploring the Possibility of O and Ne Contamination in Ulysses Observations of Interstellar Helium

Exploring the Possibility of O and Ne Contamination in Ulysses Observations of Interstellar Helium

by Wood et al.

Abstract: We explore the possibility that interstellar O and Ne may be contributing to the particle signal from the GAS instrument on Ulysses, which is generally assumed to be entirely He. Motivating this study is the recognition that an interstellar temperature higher than any previously estimated from Ulysses data could potentially resolve a discrepancy between Ulysses He measurements and those from the Interstellar Boundary Explorer (IBEX). Contamination by O and Ne could lead to Ulysses temperature measurements that are too low. We esti- mate the degree of O and Ne contamination necessary to increase the inferred Ulysses temperature to 8500 K, which would be consistent with both the Ulysses and IBEX data given the same interstellar flow speed. We find that producing the desired effect requires a heavy element contamination level of ∼ 9% of the total Ulysses/GAS signal. However, this degree of heavy element contribution is about an order of magnitude higher than expected based on our best estimates of detection efficiencies, ISM abundances, and heliospheric survival probabilities, making it unlikely that heavy element contamination is significantly affecting temperatures derived from Ulysses data.

**Fig. 3 from Wood et al. 2015**: Single map fits have been performed on synthetic Ulysses particle beam maps, computed assuming various values of R (measuring contamination of O and Ne flux), with map sampling based on 20 real observations. For each of the 20 cases, the measured temperature is plotted vs. R, with different symbols and line styles indicating different ranges of orbital phase, ϕ. (The ecliptic plane crossing is at ϕ = 0.5.)

**Fig. 4 from Wood et al. 2015**: Temperatures measured by single map fits are plotted vs. orbital phase. Measurements are shown for the actual Ulysses data, as well as for the synthetic maps made assuming R = 0.03 and R = 0.04.