2011 ARTEMIS SCIENCE NUGGETS
First lunar wake passage of ARTEMIS: Discrimination of wake effects and solar wind fluctuations by 3D hybrid simulations
by Stefan Wiehle, Technische Universität Braunschweig, Germany
Introduction
During its first lunar wake flyby on Feb 13, 2010, ARTEMIS P1 crossed the lunar wake, a region of high plasma activity behind the Moon. This region is dominated by the refilling process, where surrounding solar wind streams into the void created by the presence of the lunar body (more info available here). During this passage, the measured solar wind and especially its magnetic field showed strong fluctuations, some even appearing to be wave-like. In order to separate the effects of the lunar wake from changes in the higly dynamic solar wind, a dynamic 3D hybrid simulation was performed. In this simulation, the solar wind ions are calculated as particles affected by the Lorentz force, while the much lighter electrons are treated as charge-neutralizing, massless fluid. Unlike non-dynamic, stationary simulations, the upstream values of this simulation are dynamically modified according to solar wind data from the NASA OMNI database, shifted to the Moon's position.
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| Figure 1. Comparison between the measurements (green) and the dynamic simulation (red). The top three panels show the three magnetic field components and the bottom panels shows the solar wind density. |
Results
With this dynamic simulation, the P1 measurements could be well reproduced. A further comparison using a stationary simulation with constant solar wind parameters proved most of the observed fluctuations to be already contained in the solar wind and, thus, are not triggered by the lunar wake. Figure 1 gives a comparison between the dynamic simulation and the measurements. Due to the full 3D simulation of the Moon and the lunar wake, it could also be shown that the Moon permanently triggers the three magnetohydrodynamic (MHD) wave modes of a plasma (fast, Alfvén and slow waves). Each wave mode can be distinguished by its propagation speed and influence on the magnetic field components, depicted in Figure 2. While the circles are the result of analytical wave-propagation calculations, the color-coded planes show simulation results of the magnetic field fluctuations, in parallel and perpendicular to the background magnetic field, respectively. The simulated structures match the calculations very well, clearly indicating the presence of these wave modes in the lunar wake.
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| Figure 2. A cross section of the lunar wake from the simulation, taken at a distance of 4.8 lunar radii (~8350 km) behind the Moon. Color-coded are the perturbations of the magnetic field in the direction parallel (left panel) and perpendicular (right panel) to the background field. The overlayed black/white circles are results of analytical wave propagation calculations, depicting highest expansion starting from the lunar surface; the innermost circle being the lunar radius, follow by the slow wave, then Alfvén wave and outermost the fast wave. |
Conclusion
The simulations performed with dynamic solar wind input did show that most of the strong fluctuations are indeed solar wind related. In comparision to the solar wind magnetic field fluctuations, the effects triggered by the lunar wake are very low, although not less important. With solar winds effects separated, the simulation showed the expansion of the three MHD wave modes in the lunar wake, a result which might be proven by by the many ARTEMIS lunar wake crossings yet to come.
Reference
Wiehle, S., F. Plaschke, U. Motschmann, K.-H. Glassmeier, H.U. Auster, V. Angelopoulos, J. Mueller, H. Kriegel, E. Georgescu, J. Halekas, D.G. Sibeck, J.P. McFadden (2011), First Lunar Wake Passage of ARTEMIS: Discrimination of Wake Effects and Solar Wind Fluctuations by 3D Hybrid Simulations, Planet. Space Sci., 59(8), 661-671, doi:10.1016/j.pss.2011.01.012Biographical Note
Stefan Wiehle is a PhD student at Technische Universität Braunschweig, Germany. He started working in the group for Numerical Plasma Simulations for his BSc and has also performed simulations of the solar wind interaction with other bodies, like comets and Venus.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu