Nonthermal Ions With Partial Ring Structure in the Lunar Upstream Region

Details Nonthermal Ions With Partial Ring Structure in the Lunar Upstream Region Nonthermal Ions With Partial Ring Structure in the Lunar Upstream Region

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsm12a0840f&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #SM12A-0840

Physics

2152 Pickup Ions, 2164 Solar Wind Plasma, 2780 Solar Wind Interactions With Unmagnetized Bodies, 5421 Interactions With Particles And Fields, 6250 Moon (1221)

Scientific paper

Nonthermal ions in the lunar upstream region were observed by Ion Spectrum Analyzer (ISA) on board NOZOMI spacecraft during its lunar swing-by on Dec. 18, 1998. The spacecraft was in the solar wind during the observation and the altitude at the closest approach was about 2,800 km. The nonthermal ions have a partial ring structure in the velocity phase space, which is very similar to the pick-up ions previously observed near comets, Venus, and Mars. We examined four possible generation mechanisms of the nonthermal ions: (1) solar wind protons deflected by IMF disturbances, independent of the Moon, (2) escaped heavy ions photoionized or sputtered by the solar wind from the lunar surface, (3) protons or heavy ions ionized by energetic electron impact, charge exchange, or photoionization in the lunar exosphere, (4) deflected solar wind protons in the vicinity of the Moon. As for the species of the nonthermal ions, the ISA is a mass per charge analyzer so that it cannot distinguish the species of ions. However, we can estimate their species by analyzing their partial ring distribution in the velocity space. We concluded that they are protons from this analysis. Then, we traced trajectories of the nonthermal ions back in time following the equation of motion to estimate source locations of the nonthermal ions. The result clearly shows that most of them were from the dayside of the Moon. It also shows that the nonthermal ions had as large velocity as the solar wind ions when they were generated. With a careful examination of four possible mechanisms, we found that the mechanism (1) is ruled out because the ions emerged only when the satellite is close to the Moon. The mechanism (2) is also ruled out because the ions are not heavy ion. The mechanism (3) is also unthinkable because the mechanism requires the initial velocities of the nonthermal ions to be almost zero when they were generated. This contradicts to our result. The mechanism (4) was only left in order to explain the source location and large initial velocity. Based upon those results, we propose the following scenario for the lunar ions: however the solar wind ions have been assumed to be absorbed when they reached to the lunar surface, we suggest that some of them are deflected in the close vicinity of the Moon. Such deflected ions have large velocities due to the the bulk velocity of the solar wind and they start to move by the force of electric field and gyrate around the magnetic field. This motion forms partial ring structure with large initial velocities in the velocity space.

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