Local interstellar neutral hydrogen sampled in situ by IBEX

Lukas Saul, Peter Wurz, Diego Rodriguez, Jürgen Scheer, Eberhard Möbius, Nathan Schwadron, Harald Kucharek, Trevor Leonard, MacIej Bzowski, Stephen Fuselier, Geoff Crew, Dave McComas

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59 Scopus citations


Hydrogen gas is the dominant component of the local interstellar medium. However, owing 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 Interstellar Boundary Explorer observations. Observations are consistent with hydrogen experiencing an effective ratio of outward solar radiation pressure to inward gravitational force greater than unity (μ > 1); the temporal change observed in the local interstellar hydrogen flux can be explained with solar variability.

Original languageEnglish (US)
Article number14
JournalAstrophysical Journal, Supplement Series
Issue number2
StatePublished - 2012
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • ISM: general
  • ISM: kinematics and dynamics
  • Sun: heliosphere
  • Sun: particle emission


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