Ulysses and IBEX Constraints on the Interstellar Neutral Helium Distribution

TL;DR

This study relaxes the Maxwellian assumption for interstellar neutral helium distributions, exploring how non-Maxwellian features and heavy neutral contamination affect data interpretation from Ulysses and IBEX, leading to improved data fits.

Contribution

It introduces a non-Maxwellian analysis of interstellar helium data, considering heavy neutral contamination and velocity distribution asymmetries to better interpret observations.

Findings

Heavy neutral contamination is insufficient to explain temperature discrepancies.

Asymmetries in ISM velocity distribution improve data fit quality.

Non-Maxwellian models can reconcile differences between Ulysses and IBEX measurements.

Abstract

We relax the usual assumption of Maxwellian velocity distributions in the interstellar medium (ISM) in the analysis of neutral He particle data from Ulysses and the Interstellar Boundary Explorer (IBEX). For Ulysses, the possibility that a narrow component from heavy neutrals is contaminating the He signal is considered, which could potentially explain the lower ISM temperature measured by Ulysses compared to IBEX. The expected heavy element contribution is about an order of magnitude too small to resolve that discrepancy. For IBEX, we find that modest asymmetries in the ISM velocity distribution can potentially improve the quality of fit to the first two years of data, and perhaps improve agreement with the Ulysses measurements.