V.P52 — Alfv'en speed as cross-coupling readout

Alfven speed as cross-coupling readout: v_A^2 propto kappa(B,D) / (rho/B_0^2) = iota_tau^2 (1 - iota_tau) times the field-to-density ratio; the Alfven speed is a readout of the B-D cross-coupling strength, not an independent parameter.