Local Interstellar Neutral Hydrogen sampled in-situ by IBEX

TL;DR

This paper presents the first detailed in-situ analysis of local interstellar neutral hydrogen using IBEX, revealing directional offset, temporal variation, and the influence of solar radiation pressure on hydrogen flux.

Contribution

It provides the first detailed in-situ sampling and analysis of interstellar neutral hydrogen, highlighting its directional offset and temporal variability due to solar activity.

Findings

Hydrogen arrival direction is offset from helium.

Detected variation in hydrogen flux over two years.

Hydrogen experiences a solar radiation pressure greater than gravity.

Abstract

Hydrogen gas is the dominant component of the local interstellar medium. However, due to ionization and interaction with the heliosphere, direct sampling of neutral hydrogen in the inner heliosphere is more difficult than sampling the local interstellar neutral helium, which penetrates deep into the heliosphere. In this paper we report on the first detailed analysis of the direct sampling of neutral hydrogen from the local interstellar medium. We confirm that the arrival direction of hydrogen is offset from that of the local Helium component. We further report the discovery of a variation of the penetrating Hydrogen over the first two years of IBEX observations. Observations are consistent with hydrogen experiencing an effective ratio of outward solar radiation pressure to inward gravitational force greater than unity ({\mu}>1); the temporal change observed in the local interstellar hydrogen flux can be explained with solar variability.