An Etymological Dictionary of Astronomy and Astrophysics

A → dimensionless quantity used in → magnetohydrodynamics to describe the relative balance of
→ magnetic advection to → magnetic diffusion. It is given by:

R_m = σμ₀νLU₀,

where σ is the → conductivity of the fluid, μ₀ is the → magnetic permeability of the fluid, L is he characteristic length scale of the fluid flow, and
U₀ the characteristic velocity of the flow. A typical value for the Earth is R_m ~ 200.

See also: → magnetic; → Reynolds number.