The basic momentum-balance equation that is used to infer local
values of the momentum diffusivity () from measurements of
the rotation speed is 
Generally the densities and rotation frequencies are measured, the torque densities are calculated from a beam code, and the equation is solved for . The magnitude of the convection term is somewhat arbitrary--the choice made in Eq. 12 is that each ion convected across a flux surface carries with it the local rotation speed. The turbulence which actually drives cross-field transport might well impose a different multiplier. The terms are the radial fluxes of each ion species, which are determined separately from the particle continuity equation. The assumption has been made that all the ions rotate at the same rotation speed, which may not be valid in all regimes . Ripple damping is neglected.