Dynamic Structures of Lunar Plasmas Resulting from Moon-Solar Wind Interaction: New Findings by NOZOMI
Solar-Planetary Electromagnetism Laboratory,
Department of Geophysics,
Graduate School of Sciecne,
Kyoto University.
Submitted: 2002.12.20
Accepted: 2003.03.24
The primary subject of this thesis is the plasma environment in the vicinity of the Moon. The study is based on the in situ observations made by particle energy spectrum analyzers mounted on NOZOMI spacecraft at the time of its second lunar swing-by on Dec. 18, 1998.
The nonthermal ions were detected by Particle Spectrum Analyzer/Ion Spectrum Analyzer(PSA/ISA) on board NOZOMI. The data of 3-D velocity distribution functions of the nonthermal ions were analyzed and we found the following characteristics: the nonthermal ions had a partial ring structure in the velocity phase space, they were protons, their source location was the dayside of the Moon, and they had large velocities when they were generated.
The fact that the solar wind protons are reflected in the vicinity of the Moon implies the existence of some dynamic structure such as a small-scale shock associated with a local magnetic anomaly at the lunar surface. We propose the following scenario of the solar wind interaction with the Moon which successfully explains the observed characteristics of the nonthermal protons. When the solar wind protons are deflected by the dynamic structure in the vicinity of the Moon, the first adiabatic invariant is violated. After the deflection, they move under the force of the convection electric field and gyrate around the solar wind magnetic field. This motion forms a partial ring structure with large initial velocities in the velocity phase space.
Backstreaming electrons were also detected by Particle Spectrum Analyzer/Electron Spectrum Analyzer (PSA/ESA) during NOZOMI's second lunar swing-by. From the characteristics of the electron distribution function, we can categorize these events into two types: (1) backstreaming electrons exhibiting a velocity distribution similar to that of the solar wind electrons, but its phase space density ratio to the solar wind electrons decreases as a function of velocity; (2) backstreaming electrons which are thermalized and have a flux comparable to or dominating that of the solar wind electrons.
We considered possible source locations as well as possible mechanisms that can produce these backstreaming electrons. After careful investigations of the velocity distribution function of the electrons and the magnetic field orientation, we concluded that the origin of backstreaming electrons observed in the first event is the lunar wake region, where the electrostatic potential drop associated with the ambipolar plasma expansion reflects the solar wind electrons. The energy of the reflected electrons reaches 480 eV, so that the magnitude of ambipolar electrostatic potential is estimated to be at least 480 V.
As for the second event, the electron energy spectrum implies that the solar wind electrons have thermalized at the downstream region. There are two candidates for the thermalization mechanisms; the terrestrial bow shock and the interaction region of the solar wind ions in the downtail of the Moon. When solar wind protons passing both side of the terminator of the Moon interact with each other behind the void region, the two-stream instability can easily develop. After the thermalization of the solar wind electrons, the considerable amount of the solar wind electrons can be reflected back. Both cases can account for the observed counterstreaming electrons of the second event.
Detailed dynamic structures of lunar plasma environment are revealed by the observations by modern instruments on board NOZOMI. The ion observations proved the existence of dynamic structures in front of the Moon. The structures are related to the locally magnetized regions of lunar crustal origin. Our results motivate the theories and numerical modellings of the solar wind interaction with small-scale magnetized region, of which scale is almost of the same order to the gyro-radius of the solar wind protons.
The electron observations detected the large electrostatic potential in the lunar downstream region. The value of 480 V is larger than the value of ~40 V by numerical calculations. The discrepancy may result from some assumptions made in the calculations. Further studies with more accurate model are necessary for understanding the physics of the lunar wake.
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