Leakage at Total Field Scattered Field Box

I'm having some interesting effects after the plane wave has exited the TFSF. Although the response drops by 2 order of magnitude, there are pockets of Electric Field that stay stationary on the left and right sides of the TFSF box. Is there any way around this problem? Or is this just a consequence of using TFSF and a plane wave?

I have also have a link to a .gif file to illustrate what I am seeing: ../imgqa/eboard/EM/EM-wikx3jlyvm4.gif

If anyone has any insight, it would be greatly appreciated!