The Interstellar Neutral He haze in the heliosphere: what can we learn?

TL;DR

This paper investigates the interstellar helium haze in the heliosphere using IBEX data, exploring how low-intensity signals can reveal details about the helium distribution and departures from equilibrium, including the Warm Breeze component.

Contribution

It introduces a novel analysis of low-intensity interstellar helium signals, comparing different distribution models and highlighting the diagnostic potential of the fall peak in IBEX data.

Findings

Low-intensity signals contain important information about helium distribution.

Differences between models are most visible in specific sky regions.

The fall peak can help assess IBEX-Lo sensor thresholds.

Abstract

Neutral interstellar helium has been observed by the Interstellar Boundary Explorer (IBEX) since 2009 with a signal-to-noise ratio well above 1000. Because of the geometry of the observations, the signal observed from January to March each year is the easiest to identify. However, as we show via simulations, the portion of the signal in the range of intensities from 10^{-3} to 10^{-2} of the peak value, previously mostly left out from the analysis, may bring important information about the details of the distribution function of interstellar He gas in front of the heliosphere. In particular, these observations may inform us about possible departures of the parent interstellar He population from equilibrium. We compare the expected distribution of the signal for the canonical assumption of a single Maxwell-Boltzmann population with the distributions for a superposition of the Maxwell-Boltzmann primary population and the recently discovered Warm Breeze, and for a single primary population given by a kappa function. We identify the regions on the sky where the differences between those cases are expected to be the most visible against the background. We discuss the diagnostic potential of the fall peak of the interstellar signal, reduced by a factor of 50 due to the Compton-Getting effect but still above the detection limit of IBEX. We point out the strong energy dependence of the fall signal and suggest that searching for this signal in the data could bring an independent assessment of the low-energy measurement threshold of the IBEX-Lo sensor.