A Ulysses Detection of Secondary Helium Neutrals

TL;DR

This study detects a secondary helium neutral flow in Ulysses data, consistent with IBEX findings, and estimates its properties, highlighting the importance of flow assumptions in interpreting interstellar helium measurements.

Contribution

It provides the first detection of the secondary helium component in Ulysses data and compares its properties with IBEX measurements, emphasizing the impact of flow assumptions on temperature estimates.

Findings

Detected secondary helium flow in Ulysses data.

Measured flow parameters: V=12.8 km/s, lambda=74.4°, beta=-10.5°, T=3000 K.

Secondary component density is about 5% of the primary.

Abstract

The Interstellar Boundary EXplorer (IBEX) mission has recently studied the flow of interstellar neutral He atoms through the solar system, and discovered the existence of a secondary He flow likely originating in the outer heliosheath. We find evidence for this secondary component in Ulysses data. By coadding hundreds of Ulysses He beam maps together to maximize signal-to-noise, we identify a weak signal that is credibly associated with the secondary component. Assuming a laminar flow from infinity, we infer the following He flow parameters: V=12.8+/-1.9 km/s, lambda=74.4+/-1.8 deg, beta=-10.5+/-4.1 deg, and T=3000+/-1100 K; where lambda and beta are the ecliptic longitude and latitude direction in J2000 coordinates. The secondary component has a density that is 4.9+/-0.9% that of the primary component. These measurements are reasonably consistent with measurements from IBEX, with the exception of temperature, where our temperature is much lower than IBEX's T=9500 K. Even the higher IBEX temperature is suspiciously low compared to expectactions for the outer heliosheath source region. The implausibly low temperatures are due to the incorrect assumption of a laminar flow instead of a diverging one, given that the flow in the outer heliosheath source region will be deflecting around the heliopause. As for why the IBEX and Ulysses T values are different, difficulties with background subtraction in the Ulysses data are a potential source of concern, but the discrepancy may also be another effect of the improper laminar flow assumption, which could affect the IBEX and Ulysses analyses differently.