Upper panel: Local Alfvén speed, v_A (plus symbols), inferred from the Mach numbers in Figure 10. Dashed and dash-dotted lines refer to v_A that are modeled with the empirical electron density models of Leblanc et al. (1998) and Saito et al. (1977), respectively, together with the empirical magnetic field function of B = 2.2(r/R_⊙)⁻¹. Lower panel: Magnetic field strength B inferred from v_A in the upper panel together with the determined ρ_u. Lines in dashed and dash-dotted are radial magnetic field profiles given in Mann et al. (1999) above the quiet Sun and Dulk and McLean (1978) above an active region, respectively.