Local Interstellar Hydrogen's Disappearance at 1 Au: Four Years of IBEX in the Rising Solar Cycle

TL;DR

This study uses four years of IBEX data to show that local interstellar hydrogen at 1 AU has nearly disappeared due to increased solar activity, providing insights into heliospheric interactions.

Contribution

First direct sampling of local interstellar hydrogen at 1 AU over four years, revealing its decline and offering data to improve heliospheric models.

Findings

Hydrogen signal at 1 AU decreased by a factor of ~8 from 2012.

The hydrogen flow's longitudinal offset increased over time.

Observations serve as a benchmark for heliospheric interaction modeling.

Abstract

NASA's Interstellar Boundary Explorer (IBEX) mission has recently opened a new window on the interstellar medium (ISM) by imaging neutral atoms. One "bright" feature in the sky is the interstellar wind flowing into the solar system. Composed of remnants of stellar explosions as well as primordial gas and plasma, the ISM is by no means uniform. The interaction of the local ISM with the solar wind shapes our heliospheric environment with hydrogen being the dominant component of the very local ISM. In this paper, we report on direct sampling of the neutral hydrogen of the local ISM over four years of IBEX observations. The hydrogen wind observed at 1 AU has decreased and nearly disappeared as the solar activity has increased over the last four years; the signal at 1 AU has dropped off in 2012 by a factor of ~8 to near background levels. The longitudinal offset has also increased with time presumably due to greater radiation pressure deflecting the interstellar wind. We present longitudinal and latitudinal arrival direction measurements of the bulk flow as measured over four years beginning at near solar minimum conditions. The H distribution we observe at 1 AU is expected to be different from that outside the heliopause due to ionization, photon pressure, gravity, and filtration by interactions with heliospheric plasma populations. These observations provide an important benchmark for modeling of the global heliospheric interaction. Based on these observations we suggest a further course of scientific action to observe neutral hydrogen over a full solar cycle with IBEX.