Revisiting Ulysses Observations of Interstellar Helium

TL;DR

This study reanalyzed Ulysses data on interstellar helium flow, confirming stability over time, refining flow parameters, and estimating interstellar helium density with a novel method, while addressing discrepancies with other measurements.

Contribution

It provides a comprehensive reanalysis of Ulysses observations, refining interstellar helium flow parameters and introducing a new technique to estimate helium density from the data.

Findings

No significant change in helium flow from 1994-2007.

Refined flow velocity and direction parameters.

Estimated interstellar helium density using a novel method.

Abstract

We report the results of a comprehensive reanalysis of Ulysses observations of interstellar He atoms flowing through the solar system, the goal being to reassess the interstellar He flow vector and to search for evidence of variability in this vector. We find no evidence that the He beam seen by Ulysses changes at all from 1994-2007. The direction of flow changes by no more than ~0.3 deg and the speed by no more than ~0.3 km/s. A global fit to all acceptable He beam maps from 1994-2007 yields the following He flow parameters: V_ISM=26.08+/-0.21 km/s, lambda=75.54+/-0.19 deg, beta=-5.44+/-0.24 deg, and T=7260+/-270 K; where lambda and beta are the ecliptic longitude and latitude direction in J2000 coordinates. The flow vector is consistent with the original analysis of the Ulysses team, but our temperature is significantly higher. The higher temperature somewhat mitigates a discrepancy that exists in the He flow parameters measured by Ulysses and the Interstellar Boundary Explorer, but does not resolve it entirely. Using a novel technique to infer photoionization loss rates directly from Ulysses data, we estimate a density of n_He=0.0196+/-0.0033 cm^-3 in the interstellar medium.