Magnetic field estimates, for a M = 1 M_j planet, with T_eq = 300 K (a cold Jupiter, cyan), 1500 K (black), and 2000 K (grey), and for T_eq = 1500 K plus Ohmic heating with υ_avg = 3 km/s, but fixing B_atm = 10 G (red, see text for the motivation). The different line styles correspond to the estimates by Christensen et al. (2009) (thick lines), eq. (18) with f_ohm = 0.5, using the integral in the nominal dynamo region only (p > p_dyn = 10⁶ bar, solid lines) or in the entire convective region (dashes), and by Reiners et al. (2009), eq. (A.1) (thin lines), using the internal luminosity at p = 10⁶ bar (solid) or at p = 10³ bar (dashed).