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Physical Variables used in MHD Simulations

The basic plasma and electrodynamic field variables (written by each MHD model) are:

• Mass density Rho or particle number density N.
• Plasma pressure P.
• Plasma velocity V with its three components V_x, V_y and V_z.
• Magnetic field B with its three components B_x, B_y and B_z.
• B1 is the difference between B and the Earth's dipole field (components B1_x, B1_y, B1_z).
  B1 is used to see small variations of B near the Earth.
• BA is the "B-field anomaly", the difference between the total field strength B and the strength of the Earth's dipole field B_dipole.
• Electric current density J with its three components J_x, J_y and J_z.

Primary variables witten occasionally or by some models only:

• Internal Energy En. (P/(γ-1) with γ="ratio of specific heats", usually 5/3).
• Resistivity Eta (some runs).

Derived quantities
Note: everything listed here and below is calculated by CCMC.
Use is at your own risk. Please report errors to us.

• B-parallel electric current density J_par with its three components J_par_x, J_par_y and J_par_z.
  Note: flow line tracings do not make much sense with this vector!
• Electric field E with its three components E_x, E_y and E_z.
• Plasma-β: "beta"=P/(B²/2μ₀).
• Plasma Frequency: "omega"
  • ω_pi = 1.32*10³*(N [cm^-3])^1/2 - ion plasma frequency
  • ω_pe = 5.64*10⁴*(N [cm^-3])^1/2 - electron plasma frequency
  We assume N=N_e=N_i (electron-proton plasma).

The underscore ("_") indicates subscript level in variable name used in plots.

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Gradients and divergences (magnetosphere models only at this time):

Gradients of selected scalar variables (e.g., gradN, gradT, gradP) and divergences of selected vectors (e.g., div.V, div.ExB) can be requested for visualization and ASCII data output.
A certain discrepancy from expected values can be expected since divergences and gradients are computed based on a finite differencing of data that may have been interpolated before computing the gradient or divergence.
Vector values in some models may be defined on staggered meshes. For visualization and the computation of derived quantities (such as ExB) all vector variables are interpolated to cell center positions (where plasma parameters such as N, P, T already reside).

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Quantities scaled with radial distance in solar coronal and heliospheric models:

Coordinates: r,lon,lat for MAS, ENLIL and x,y,z for SWMF

• Scaled with r²:
  N, P, B_r, JxB, E.J (if currents J are available)
• Scaled with r¹:
  B (B_lon,B_lat or B_x,B_y,B_z),
  E (E_r,E_lon,E_lat or E_x,E_y,E_z),
  J (J_r,J_lon,J_lat or J_x,J_y,J_z),
  J_par(J_par_r,J_par_lon,J_par_lat or J_par_x,J_par_y,J_par_z)
• Not scaled:
  T, beta, S (entropy), V (V_r,V_lon,V_lat or V_x,V_y,V_z)

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Last updated: Nov. 19, 2007 - Lutz Rastätter

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