Alfven number formula |
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\( Ma \;=\; \dfrac{ v }{ v_a }\) (Alfven Number) \( v \;=\; Ma \cdot v_a \) \( v_a \;=\; \dfrac{ v }{ Ma }\) |
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| Symbol | English | Metric |
| \( Ma \) = Alfven Number | \( dimensionless \) | \( dimensionless \) |
| \( v \) = Velocity of Fluid or Plasma | \(ft \;/\; sec\) | \(m \;/\; s\) |
| \( v_a \) = \(\dfrac{B}{\sqrt{\mu_0 \cdot \rho} }\) is the Alfven Speed | \(ft \;/\; sec\) | \(m \;/\; s\) |
| \( B \) = Magnetic Field Strength | \(T\) | \(kg\;/\;s^2-A\) |
| \( \mu_0 \) (Greek symbol mu) = Permeability of Free Space | \(ft^2\) | \(m^2\) |
| \( \rho \) (Greek symbol rho) = Density of Fluid or Plasma | \(lbm \;/\; ft^3\) | \(kg \;/\; m^3\) |
Alfven number, abbreviated as Ma, a dimensionless number, is used in magnetohydrodynamics (MHD) to describe the ratio of the flow velocity of a plasma or fluid to the Alfvén speed. The Alfvén speed is the speed at which magnetic disturbances propagate through a magnetized plasma, and it depends on the magnetic field strength and the medium's density.
Alfven Number Interpretation
- Low Alfven Number (Ma < 1) - The flow is sub-Alfvenic, the fluid moves slower than the Alfvén speed, and magnetic effects dominate.
- Intermediate Alfven Number (Ma ≈ 1) - The flow is Alfvenic, and the fluid fluid velocity matches the Alfvén speed.
- High Alfven Number (Ma > 1) - The flow is super-Alfvenic, indicating that the fluid moves faster than the Alfvén speed, and inertial forces dominate over magnetic forces.
The Alfvén number is used in astrophysics and plasma physics, particularly when studying phenomena like solar winds, stellar interiors, or fusion plasmas, where magnetic fields and fluid dynamics interact.
