The fields associated with traces over a conducting plane are located under the traces. At frequencies below a kilohertz, a circuit analysis will show that the series inductive reactance between points is lower than the resistance between points. At these low frequencies, current patterns are controlled by conductor resistances. The field required to move current in a ground plane at dc is very small. As a result, the E field pattern at dc is very similar to the pattern at 1 MHz. If the resistivity of the ground plane were zero then no field would be required to move current and the pattern under the trace would not depend on frequency.