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Non-magnetic Planets Yingjuan Ma, Andrew Nagy, Gabor Toth, Igor Sololov, KC Hansen, Darren DeZeeuw, Dalal Najib, Chuanfei Dong, Steve Bougher SWMF User Meeting Oct. 14, 2014
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Introduction Both Venus and Mars do not have global internal magnetic field but with substantial atmosphere. As a result, solar wind plasma flow interact more directly with the atmosphere/ionosphere system as compared with Earth. The upstream plasma flow interact not only through electric-magnetic forces with the ionosphere, but also through collisions with neutral atmosphere. 2
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Interaction with Ionosphere
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Ionosphere of Venus and Mars
From Chen et al., 1978 From Nagy et al., 1980
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Interaction with Atmosphere -Collisions are important
Elastic collisions: momentum and energy loss of the plasma Inelastic collisions Photoionization: A + h A+ + e Increase the plasma number density Decrease the average flow speed and temperature Charge exchange: A + B+ A+ + B Usually increase the plasma mass density Decease the total momentum and energy of the plasma Recombination: A+ + e A Decease the number density, momentum and energy of the plasma
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Multi-Species Single-fluid MHD Equations
Continuity Equations: Momentum Equation: ( ) Density change caused by chemical reactions Momentum loss due to ion-neutral elastic collisions Momentum loss due to chemical reactions 6
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Multi-Species Single-fluid MHD Equations (2)
Magnetic Induction Equation: Energy Equation ( ) Energy change due to ion-neutral elastic collisions Energy change due to chemical reactions 7
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Numerical Method (BATSRUS)
2nd order finite-volume approach Flux functions based on approximate Riemann solver from Linde et al. [2002] 2-stage explicit update scheme with point-implicit scheme for source terms to ensure stabilities.
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Simulation Details Four ion species: H+, O2+, O+, CO2+; two background neutrals: CO2, O; and eight chemical reactions. Spherical grids: Computational domain: –24RV ≤ X ≤ 8 RV, –16RV ≤ Y, Z ≤ 16 RV ; Radial resolution varies from 5 km in the ionosphere to 3000 km further away; Angular resolution is 2.50 ; 5 million cells, ~5,000 CPU hours. Inner Boundary Conditions Inner boundary at 100 km; [O2+] , [O+] and [CO2+] are in photochemical equilibrium (SZA and optical depth considered); Absorbing boundary condition for U and B. Illustration of the grid system used in the calculation 9
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Examples Simulation Results of Venus Ma et al., 2013
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1D subsolar plots of densities, magnetic field and velocity.
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B Cleaning |B| in the XY plane with hyperbolic B cleaning 12
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1D subsolar plots of densities, magnetic field and velocity for cases (without and with hyperbolic B cleaning.
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Simulation Results of Mars (Ma et al., 2014)
B=B0, where B0 is the crustal magnetic field (60-order spherical harmonic model of Arkani-Hamed [2001]) 14 14 14
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Effects Diurnal Rotation of the Crustal Field
B and Field lines Crustal Field (B0) As the planet rotates, the size and shape of the obstacle to the solar wind varies, as a results, the induced magnetic field also varies with time.
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Comparison with MGS observations on May 16, 2005
*Overall good agreement between model results and MGS observations. *The agreement is the best for B magnitude (corr. Coeff =0.88, RMSE = 10.9 nT). *The corr. Coeff for components of magnetic field is not as good mostly due to IMF direction change during the day. IMF condition used in the MHD model BX =1.6 nT, BY=-2.5 nT 16 16 16
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Zoom in view of the comparison with MGS observations on May 16, 2005, over-plotted with local time.
*Good agreement near strong crustal field region indicates that the crustal field model included in the MHD model is quite accurate. *Around dayside weak crustal field region, the induced field is needed to fit with the data. *In some region, it is hard to distinguish what is the cause for the discrepancy (IMF, crustal source, or model limitation). 17 17 17
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Multi-Fluid MHD Model (Najib et al., 2011)
Continuity equation Momentum equation Pressure equation Magnetic Induction equation where the charge-averaged ion-velocity, and ue are defined as: Causes flow separation in convection electric field direction 18 18 18 18
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Venus vs Mars (Multi-fluid model)
The multi-fluid effect is much stronger at Mars than at Venus. Proton gyroradius 0.07RV at Venus, 0.4 RM at Mars. The crustal field is not included for Mars for comparison. E Mars E
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Summary BATSRUS is the best existing tool in simulating plasma interaction with non-magnetic planets. Future work Improve efficiency for multi-fluid MHD code; Couple between SPICE and BATSRUS; Couple between BATSRUS with MGITM; Extend the simulation domain inside the planet to take into account effects of subsurface conducting layer. 20 20 20
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Thank You 21 21 21
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