Kinetic simulations of finite gyroradius effects in the lunar plasma environment on global, meso, and microscales

摘要

The recent in situ particle measurements near the Moon by Chandrayaan-1 and Kaguya missions as well as the earlier observation by the Lunar Prospector have shown that the Moon-solar wind interaction is more complicated than believed earlier. The new observations have arisen the need for a detailed modelling of the near surface plasma-surface processes and regions near the lunar magnetic anomalies. Especially, interpretation of ion, electron, and energetic neutral atoms (ENA) observations have shown that the plasma cannot be treated as a single fluid but that kinetic effects have to be taken into account. We have studied the kinetic effects and, especially, the role of finite gyro-radius effects at the Moon by kinetic plasma simulations at three different length-scales which exist in the Moon-solar wind interaction. The solar wind interaction with a magnetic dipole, which mimics the lunar magnetic anomalies in this study, is investigated by a 3D self-consistent hybrid model (HYB-Moon) where protons are particles and electrons form a charge neutralizing mass less fluid. This study shows that the particle flux and density and the bulk velocity of the solar wind protons that hit the lunar surface just above the dipole are decreased compared to their undisturbed values. In addition, a particle "halo" region was identified in the simulation, a region around the dipole where the proton density and the particle flux are higher than in the solar wind, qualitatively in agreement with energetic hydrogen atom observations made by the Chandrayaan-1 mission. The near surface plasma within the magnetic anomaly within a Debye sheath is studied by an electromagnetic Particle-in-Cell, PIC, simulation (HYB-es). In the PIC simulation both ions and electrons are treated as particles. Further, we assume in the PIC simulation that the magnetic anomaly blocks away all solar wind particles and the simulation contains only photo-electrons. The analysis shows that the increased magnetic field decreases the strength of the electric potential and results in a thinner potential sheath than without the magnetic field. Overall, the simulations give support for the suggestions that kinetic effects play an important role on the properties of the lunar plasma environment.