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A Global Hybrid Simulation Study of the Solar Wind Interaction with the Moon

A Global Hybrid Simulation Study of the Solar Wind Interaction with the Moon. David Schriver ESS 265 – June 2, 2005. Solar Wind – Plasma from the Sun. Goal and Approach. Examine global kinetic aspects of the solar wind interaction with the Moon Moon has no internal dipole and no ionosphere

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A Global Hybrid Simulation Study of the Solar Wind Interaction with the Moon

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  1. A Global Hybrid Simulation Study of the Solar Wind Interaction with the Moon David Schriver ESS 265 – June 2, 2005

  2. Solar Wind – Plasma from the Sun

  3. Goal and Approach • Examine global kinetic aspects of the solar wind interaction with the Moon • Moon has no internal dipole and no ionosphere • wake-tail forms on the nightside • Use hybrid simulations of solar wind flow over a non-conducting, unmagnetized object • include ion kinetic effects • invoke realistic spatial scales and parameters

  4. Global Simulation Techniques • Magnetohydrodynamic (MHD) • 3D fluid modeling on global scales • does not include kinetic effects • Particle in cell (PIC) • includes kinetics for both electrons and ions • requires unrealistic parameters (i.e., mass ratio) • Hybrid • includes ion kinetics (fluid electrons) • use realistic parameters for some global systems • does not include electron kinetics

  5. Hybrid Code Methodology • Ion equations (full particles): • Electron equations (massless fluid): let me = 0 and ne = ni • Field equations: • Modified Ohm’s law:

  6. Hybrid Code Normalization • Spatial scale – ion inertial length i = c/wpi 102 km (solar wind at 1 AU, n = 5 cm-3) • Time scale – ion gyrofrequency i = qB/mic 12 rad/s; fci-1 0.5 s (solar wind at 1 AU, B = 5 nT) • Velocity scale – Alfvén velocity vA = i i 51 km/s (sound speed  21 km/s, Te = 5 eV) Allows small global systems to be simulated on parallel supercomputers (i.e., Moon, Mercury, Mars, etc.)

  7. Earth’s Moon radius: 1738 km orbit: 59.6 RE period: 28 days atmosphere: none magnetic field: no internal dipole (however, surface fields exist with B ~ 1-100 nT) interior: essentially non-conducting

  8. Solar Wind – Moon Interaction • Lunar surface absorbs particles on dayside • lack of atmosphere eliminates local lunar plasma source • Solar wind IMF diffuses through lunar interior • crustal magnetic fields on lunar surface may form mini-magnetospheres, but effects are localized • Plasma cavity forms on nightside region • examine structure of the wake-tail • understand plasma refilling processes

  9. Lunar Prospector Data

  10. Wind flyby summary [Bosqued et al., 1996]

  11. THE LUNAR PLASMA WAKE… [1996]

  12. Plasma waves during flyby [Farrell et al., 1996]

  13. Refilling of Moon’s Wake-Tail • Kinetic processes in Moon’s wake tail are observed: • streaming and anisotropic ion distributions • plasma waves of various types

  14. Lunar Wake-Tail Refilling Studies • Fluid interaction with obstacle • rarefaction and trailing shock wave form down tail [Michel, 1968; Wolf, 1968; Spreiteret al., 1970] • Particle studies • ions removed along Sun-Moon line [Whang, 1968] • electrons removed along the IMF direction [Bale et al., 1997] • Kinetic studies • 1D PIC simulations show streaming and charge separation instabilities [Farrell et al., 1997; Birch and Chapman, 2001] • few global kinetic self-consistent studies [e.g. Lipatov, 2002]

  15. Hybrid Simulation Setup • code: current advance method – cyclic leapfrog (CAM-CL) [Matthews, 1994] • 2D system size: Lx  Ly= 3200Dx 1280Dy 53 RL 26 RL • grid spacing: Dx=0.2 i and Dy=0.25 i (i = c/wpi = RL/12) • time step: Dt= 0.005 Wci-1 • solar wind speed: vsw = 6 vA (~ 400 km/s); plasma beta: bi = 0.6 and be = 0.4 • uniform constant resistivity: = 0.02vA/Wci • IMF direction (with respect to the solar wind flow):  = 45o and 90o

  16. Density Profiles  = 90o  = 45o 0 10 20 30 x/RL

  17.  = 45o Ion phase space perp. parallel

  18. T/T|| anisotropy contours  = 45o Bfluctuations  = 45o

  19. B FFT Spectrum (28RL < x < 40RL; 4.4RL < y < 4.4RL)

  20. WIND Lunar flyby ~25 RL wave spectra ion energy B components |B| density

  21. Conclusions • Cavity refilling is described by a magnetized plasma to vacuum expansion; the electron pressure gradient at the cavity’s edge provides a parallel electric field • The rate of the plasma refilling process depends on the orientation of the IMF • The density cavity is filled with counterstreaming ion beams and highly anisotropic plasma • Left-hand polarized electromagnetic VLF waves are generated in the region 28RL < x < 40RL, probably generated by an anisotropy and/or heat flux instability

  22. Future Research • Perform more two and three-dimensional Moon runs • vary solar wind speed, density, IMF intensity • use upstream solar wind data to drive simulation • examine plasma environment in Earth’s magnetosphere • Add surface magnetic field sources • examine the formation and effects of mini-magnetospheres (surface shielding) • Simulations of Mercury’s magnetosphere • preparation for Messenger, Bepi-Colombo missions

  23. Density Comparison:Data with Simulations

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