Evidence for Asymmetry in the Velocity Distribution of the Interstellar Neutral Helium Flow Observed by IBEX and Ulysses

Abstract

We use observations from the Interstellar Boundary Explorer ( IBEX ) and Ulysses to explore the possibility that the interstellar neutral helium flowing through the inner solar system possesses an intrinsic non-Maxwellian velocity distribution. In fitting the IBEX and Ulysses data, we experiment with both a kappa distribution and a bi-Maxwellian, instead of the usual Maxwellian assumption. The kappa distribution does not improve the quality of fit to either the IBEX or Ulysses data, and we find lower limits to the kappa parameter of κ  > 12.1 and κ  > 6.0 from the IBEX and Ulysses analyses, respectively. In contrast, we do find evidence that a bi-Maxwellian improves fit quality. For IBEX , there is a clear preferred bi-Maxwellian solution with T ⊥ / T ∥  = 0.62 ± 0.11 oriented about an axis direction with ecliptic coordinates ( λ axis , b axis ) = (57.°2 ± 8.°9, −1.°6 ± 5.°9). The Ulysses data provide support for this result, albeit with lower statistical significance. The axis direction is close to the interstellar medium (ISM) flow direction, in a heliocentric rest frame, and is therefore unlikely to be indicative of velocity distribution asymmetries intrinsic to the ISM. It is far more likely that these results indicate the presence of asymmetries induced by interactions in the outer heliosphere.