Key Questions of Solar Wind–Moon Interaction

Abstract

Key questions on solar wind–Moon interaction are reviewed. As the nearest celestial body to Earth, Moon’s space environment is distinctive to Earth’s mainly because of lack of a significant atmosphere/ionosphere and a global magnetic field. From a global respective, solar wind can bombard its surface, and the solar wind materials cumulated in the soil record the evolution of the Solar System. 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 ground. 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. The 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.

High-energy particle observations from the Moon

“…During more than 3/4 of its orbit around the Earth the Moon is directly exposed to the solar wind. Lacking a global intrinsic magnetic field and without a collisional atmosphere, solar wind and solar energetic particles (SEPs) arrive nearly without any deviation or absorption and bombard the Moon's surface, interacting with the lunar regolith and the tenuous lunar exosphere [1][2][3][4][5]. A similar phenomenon occurs also with the galactic cosmic rays (GCRs), which have fluxes and energy spectra representative of interplanetary space [6].…”