To calculate the group velocity of Alfvén waves:
Measure the strength of the magnetic field B.
Measure the density of the plasma ρ.
Divide the magnetic field by the square root of the density multiplied by the magnetic permeability of the vacuum, µ0:
v = B/sqrt(µ0 · ρ)
That's it! Remember that Alfvén waves propagate in the direction of the magnetic field. Read more