In situ observations of interstellar neutral (ISN) helium atoms by the
IBEX-Lo instrument onboard the Interstellar Boundary Explorer (IBEX) mission
are used to determine the velocity and temperature of the pristine very local
interstellar medium (VLISM). Most ISN helium atoms penetrating the heliosphere,
known as the primary population, originate in the pristine VLISM. As the
primary atoms travel through the outer heliosheath, they charge exchange with
He+ ions in slowed and compressed plasma creating the secondary population.
With more than 2.4 million ISN helium atoms sampled by IBEX during ISN seasons
2009-2020, we compare the observations with predictions of a parametrized model
of ISN helium transport in the heliosphere. We account for the filtration of
ISN helium atoms at the heliospheric boundaries by charge exchange and elastic
collisions. We examine the sensitivity of the ISN helium fluxes to the
interstellar conditions described by the pristine VLISM velocity, temperature,
magnetic field, and composition. We show that comprehensive modeling of the
filtration processes is critical for interpreting ISN helium observations, as
the change in the derived VLISM conditions exceeds the statistical
uncertainties when accounting for these effects. The pristine VLISM parameters
found by this analysis are the flow speed (26.6 km s−1), inflow direction
in ecliptic coordinates (255.7∘, 5.04∘), temperature (7350 K),
and B-V plane inclination to the ecliptic plane (53.7∘). The derived
pristine VLISM He+ density is 9.7×103 cm−3. Additionally, we
show a strong correlation between the interstellar plasma density and magnetic
field strength deduced from these observations.Comment: 13 pages, 3 figures, 2 tables, accepted for publication in Ap