Research Highlight

Tracing Sun's protons near the Moon

doi:10.1038/nindia.2013.99 Published online 23 July 2013

Researchers have gained new insights into how protons from the solar wind travel to the Moon's surface and are scattered back into space. They studied data from sophisticated sensors onboard Chandrayaan-1, India's first unmanned lunar probe, to get these insights. The study will improve our understanding of the interaction between the solar wind and the lunar surface.

The solar wind is a continuous flow of plasma from the Sun that mainly consists of hydrogen and helium ions with traces of heavier ions. This wind interacts with obstacles in its flow such as planetary bodies and moons. Because the Moon has a surface-bound exosphere but lacks a global magnetic field, the solar wind interacts directly with the lunar surface.

This interaction neutralizes most of the solar-wind ions. Atoms ejected from the lunar surface due to the interaction have sufficient energy to be classified as energetic neutral atoms (ENAs). It is essential to observe these ENAs to better understand the interaction between the solar wind and the lunar surface. However, previous studies had not provided a clear picture of this interaction.

The researchers analyzed data from the Sub-keV Atom Reflecting Analyser (SARA) experiment aboard Chandrayaan-1. It revealed that almost 20% of incident solar-wind ions are back-scattered from the lunar surface as ENAs. In addition, the researchers observed protons in the near-lunar region (100–200 km from the surface) that came from the tail of the solar wind.

Their findings contradict the previous assumption that the lunar surface almost completely absorbs the solar wind. This observation is relevant for other bodies in the solar system that lack an atmosphere.


  1. Dhanya, M. B. et al. Proton entry into the near-lunar plasma wake for magnetic field aligned flow. Geophys. Res. Lett. 40, 2913-2917 (2013) | Article |