Paper detail

Neutral interstellar helium parameters based on Ulysses/GAS and IBEX-Lo observations: what are the reasons for the differences?

Recent analysis of the interstellar helium fluxes measured in 2009-2010 at Earth orbit by the Interstellar Boundary Explorer (IBEX) has suggested that the interstellar velocity (both direction and magnitude) is inconsistent with that derived previously from Ulysses/GAS observations made in the period from 1990 to 2002 at 1.5-5.5 AU from the Sun. Both results are model-dependent and models that were used in the analyses are different. In this paper, we perform an analysis of the Uysses/GAS and IBEX-Lo data using our state-of-the-art 3D time-dependent kinetic model of interstellar atoms in the heliosphere. For the first time we analyze Ulysses/GAS data from year 2007, the closest available Ulysses/GAS observations in time to the IBEX observations. We show that the interstellar velocity derived from the Ulysses 2007 data is consistent with previous Ulysses results and does not agree with the velocity derived from IBEX. This conclusion is very robust since, as is shown in the paper, it does not depend on the ionization rates adopted in theoretical models. We conclude that Ulysses data are not consistent with the new LISM velocity vector from IBEX. In contrast, IBEX data, in principle, could be explained with the LISM velocity vector derived from the Ulysses data. This is possible for the models with the interstellar temperature increased from 6300 K to 9000 K. There is a need to perform further study of possible reasons for the broadening of the helium signal core measured by IBEX. This could be an instrumental effect or due to unconsidered physical processes.