Vector field cross product

[Gribouillis]

I think the problem comes from the fact that By and Bz have shapes (100, 100, 100) and Bx is a number. Same with E.

It works for me with

Ex = ...
Ez = ...
Ey = np.zeros_like(Ex)
...
By = ...
Bx = np.zeros_like(By)
...
np.cross(E, B, axis=0)

The result has shape (3, 100, 100, 100). I don't know if this is what you want.