Reflection of solar wind hydrogen from the lunar surface

H. O. Funsten, F. Allegrini, P. A. Bochsler, S. A. Fuselier, M. Gruntman, K. Henderson, P. H. Janzen, R. E. Johnson, B. A. Larsen, D. J. Lawrence, D. J. McComas, E. Möbius, D. B. Reisenfeld, D. Rodríguez, N. A. Schwadron, P. Wurz

Research output: Contribution to journalArticlepeer-review

30 Scopus citations


The solar wind continuously flows out from the Sun and directly interacts with the surfaces of dust and airless planetary bodies throughout the solar system. A significant fraction of solar wind ions reflect from an object's surface as energetic neutral atoms (ENAs). ENA emission from the Moon was first observed during commissioning of the Interstellar Boundary Explorer (IBEX) mission on 3 December 2008. We present the analysis of 10 additional IBEX observations of the Moon while it was illuminated by the solar wind. For the viewing geometry and energy range (> 250 eV) of the IBEX-Hi ENA imager, we find that the spectral shape of the ENA emission from the Moon is well-represented by a linearly decreasing flux with increasing energy. The fraction of the incident solar wind ions reflected as ENAs, which is the ENA albedo and defined quantitatively as the ENA reflection coefficient R N, depends on the incident solar wind speed, ranging from ~0.2 for slow solar wind to ~0.08 for fast solar wind. The average energy per incident solar wind ion that is reflected to space is 30 eV for slow solar wind and 45 eV for fast solar wind. Once ionized, these ENAs can become pickup ions in the solar wind with a unique spectral signature that reaches 3vSW. These results apply beyond the solar system; the reflection process heats plasmas that have significant bulk flow relative to interstellar dust and cools plasmas having no net bulk flow relative to the dust. Key PointsUp to 20% of solar wind protons impacting the Moon are reflected as ENAsThe reflection probability and the average ENA energy depend on solar wind speedReflected ENAs can become PUIs and a suprathermal component of the solar wind

Original languageEnglish (US)
Pages (from-to)292-305
Number of pages14
JournalJournal of Geophysical Research: Planets
Issue number2
StatePublished - Feb 2013
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science


  • ENA
  • Moon
  • PUIs
  • albedo
  • reflection
  • solar wind


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