a: midplane gas pressure P_gas, 0 between 0.04 AU and 2 AU for two different times during an FU Ori like outburst. b: midplane pressure gradient force −∇P_gas, 0 relative to the gravitational force is plotted. The radially localized changes in sign occur at the positions of the ionization fronts running inwards and outwards. The deviation from the Keplerian angular velocity is shown in (c).