high voltage – COMSOL Simulation E-Field Results for AC & DC Difference

The top electrode is at 100kV.

conductivity air = 8e-15S/m

conductivity insulator = 1e-12,1e-14,1e-16,1e-18 S/m

permitivity insulator = 4.8

conductivity electrode = 5.96e7S/m

Both the above picture show the surface E-Field of the insulator at different insulator conductivities

I don’t understand why do the graph for AC is same for all the conductivity, but for DC the graph is different. Is there a reason for this, also is there a formula which can explain this effect,