Many small-scale remanent magnetic fields are scattered over the lunar surface and, just as planetary magnetic fields protect planets, they are believed to divert the incident solar wind and shield the local lunar surface beneath, thus producing unique local surface environment that is critical to activities of human beings/facilities, thus providing unique landing sites to explore the origins of lunar swirls and remanent magnetic fields.

Evidences have hinted that this local interaction, however, may be also distinct with the interacting scenario on planets, and the specific process has not been revealed because of lack of in situ observations in the near-Moon space or on the lunar surface.

The global and local solar wind interactions of the Moon represent 2 types of characteristic interaction of celestial bodies with stellar wind in deep space, i.e., the interactions of nonmagnetized bodies and of small-scale magnetized bodies, both of which may occur on asteroids and Mars.

These deep-space celestial bodies, either difficult or impossible to reach for human beings or artificial satellites, are hard to measure, and the exploration of the Moon can reveal the mystery of stellar wind interaction on these bodies. In a review article recently published in Space: Science and Technology, scholars from Chinese Academy of Sciences and Beihang University reviewed Key questions on solar wind-Moon interaction.

First, authors introduce the background environment and properties of the Moon (schemed in Fig. 1). The Sun continually releases energy into interplanetary space in two forms of ejecting particles and emitting radiations, which stirs up the space environment of the entire Solar System. The Sun ejects the magnetized plasma flow, which is solar wind composed of the oppositely charged particles, i.e., ions and electrons.

The difference in mass between protons and electrons is huge, which makes the solar wind a complicated material with multiple scales. The solar wind interaction of the Moon consists of processes across multiple plasma scales, thus resulting in the complicated Moon's space environment and solar wind interaction. The Sun also emits wide frequency-range radiations. About half of the solar radiation energy is confined within the visible waveband; however, the other radiations are key to the space environment around all kind of Solar System objects.

The properties of the Moon itself also influence the solar wind interaction, and they are mainly the electrical conductivity and magnetism, where interdisciplinary studies between space physics, geophysics, and even geology are involved. As for internal structures, evidence shows that the Moon as a whole is not a good conductor.

As for lunar magnetic fields, the Moon has been thought to have no global intrinsic magnetic field and, thus, have no lunar magnetosphere; but many small-scale remanent magnetic fields scattered over the lunar surface. As for the lunar atmosphere, a very thin atmosphere is present, and there is no a significant lunar ionosphere. As for the lunar surface, the interior of the Moon is covered the regolith layer, the outmost layer with a depth that can be as large as tens of kilometers. Lunar dust is another term frequently occurring when discussing the lunar surface environment, whose dynamics is controlled by the electric and magnetic fields.

Research Report:Key Questions of Solar Wind-Moon Interaction

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