A Global Hybrid Simulation Study of the Solar Wind Interaction with the Moon
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A Global Hybrid Simulation Study of the Solar Wind
Interaction with the Moon
David Schriver
ESS 265 – June 2, 2005
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Solar Wind – Plasma from the Sun
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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
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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
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Hybrid Code Methodology• Ion equations (full particles):
• Electron equations (massless fluid):
let me = 0 and ne = ni
• Field equations:
• Modified Ohm’s law:
ii
dt
dv
r ][ B v E
v i
i
ii
m
q
dt
d
E B
tJ B o
eBee TknP eeee
ee Pendt
dmn B J E
u
B BJBB
E )()(1
eioe
Pen
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Hybrid Code Normalization
• Spatial scale – ion inertial length i = c/pi 102 km(solar wind at 1 AU, n = 5 cm-3)
• Time scale – ion gyrofrequencyi = qB/mic 12 rad/s; fci
-1 0.5 s(solar wind at 1 AU, B = 5 nT)
• Velocity scale – Alfvén velocityvA = 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.)
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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
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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
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Lunar Prospector Data
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Wind flyby summary [Bosqued et al., 1996]
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THE LUNAR PLASMA WAKE… [1996]
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Plasma waves during flyby [Farrell et al., 1996]
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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
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Lunar Wake-Tail Refilling Studies
• Fluid interaction with obstacle– rarefaction and trailing shock wave form down tail [Michel,
1968; Wolf, 1968; Spreiter et 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]
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Hybrid Simulation Setup
• code: current advance method – cyclic leapfrog (CAM-CL) [Matthews, 1994]• 2D system size: Lx Ly= 3200x 1280y53 RL 26 RL
• grid spacing: x = 0.2 i and y = 0.25 i (i = c/pi = RL/12)
• time step: t = 0.005 ci-1
• solar wind speed: vsw = 6 vA (~ 400 km/s); plasma beta: i = 0.6 and e = 0.4
• uniform constant resistivity: = 0.02vA/ci
• IMF direction (with respect to the solar wind flow): = 45o and 90o
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Density Profiles
= 90o
= 45o
0 10 20 30 x/RL
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Ion phase space
perp.
parallel
= 45o
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B fluctuations
T/T|| anisotropy contours
= 45o
= 45o
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B FFT Spectrum (28RL < x < 40RL; 4.4RL < y < 4.4RL )
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WIND Lunar flyby ~25 RL
wave spectra
ion energy
B components
|B|
density
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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
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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
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Density Comparison:Data with Simulations