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TOP > Report & Column > The Forefront of Space Science > 2009 > Moon and Beyond Plasma Environment around the Moon Explored by KAGUYA

The Forefront of Space Science

Mystery of Solar Coronal Heating Being Explored by Satellites
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Fig. 2 is a so-called E-t spectrogram of ions observed by MAP-PACE. Horizontal axis is time while vertical axis is ionsEenergy. The amount of ions is expressed in color. Looking at the IEA data, a strong ion flux appears every two hours. This is because KAGUYA takes about two hours to go around the Moon.

The data shown in Fig. 2 were retrieved when the Moon was in the interplanetary space away from the geomagnetosphere. The strong ion fluxes observed by IEA are the forementioned solar wind. As the figure indicates, the solar wind of the day has energy of about 2keV. Since the main component of solar wind is hydrogen nuclei (H+), its velocity is about 600km per sec. In Fig. 2, we also see ions that are the solar-wind reflected/scattered on the lunar surface. These ions were first discovered by MAP-PACE. When IEA observed the solar wind, IMA also observed ions that were weaker than those in the solar wind. The spectrograms show that the energy of the ions observed by IMA is lower than those of the solar wind observed by IEA. Since IMA covers the hemisphere of the lunar-surface side, these ions are obviously flying from the Moon. Data similar to these were also retrieved on other days. Since the energy of these ions is always slightly lower than those of the solar wind, we can conclude that these ions are scattered or reflected from the collision of the solar wind with the lunar surface.

In the past, it was believed that solar wind ions which collided with the Moon were absorbed into its surface. We were therefore surprised that ions in measurable quantities were returned from the lunar surface. We investigated how many ions were reflected and scattered on the lunar surface. The answer was about 0.1 to 1% of the incident solar wind. Since IMA is a mass analyzer, we were able to acquire information about the species of ions observed. The result showed that the ions observed by IMA were hydrogen nuclei (H+). The solar wind also contains helium nuclei (He++), second in abundance after hydrogen nuclei. The solar wind on the observed day contained many helium nuclei, but none observed in the ions coming from the Moon. We are unable to explain why helium nuclei originally contained in solar wind are absent from the ions reflected/scattered on the lunar surface. We guess that it is because of the difference in ionization ratios when ions collide with material.

We suppose that solar wind reflected/scattered on the lunar surface contains information on surface condition, etc., of the Moon. Accordingly, we expect that, by matching ions reflected/scattered ions to locations of reflection/scattering, we can perform “remoteEexploration of the lunar surface. We also discovered that ions reflected/scattered on the surface are accelerated in the solar wind’s electric field by up to three times the solar-wind velocity. Most accelerated ions escape from the Moon, but, surprisingly, we found that some accelerated ions entered the Moon’s nightside and reached the surface. As discussed above, KAGUYA’s observations revealed for the first time that electrons and ions in the solar wind collide directly with the lunar surface and substantially affect the plasma environment around the Moon.


Figure 2
Figure 2. Ion data retrieved from two satellite orbits of the Moon provided by the Ion Energy Analyzer (IEA) and Ion Mass Analyzer (IMA).
Solar wind ions such as hydrogen nuclei (H+) and helium nuclei (He++) are observed in the dayside of the Moon. Accelerated ions and solar wind-reflected/scattered ions on the surface are observed in the direction of the Moon.


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