The fast particles in the electron foreshocks propagate along the magnetic field lines, so that the effect of low frequency magnetic fluctuations is to create both a broadening and a fine structure of the foreshock upstream boundary. This is studied by means of a 3-D numerical simulation of turbulent magnetic fields. Applications to Venus, Earth, and Mars magnetic foreshocks are done. It is found that a spacecraft can be magnetically connected to the bow shock even when it is upstream of the nominal magnetic foreshock boundary, and that the mixing of magnetic field lines can give an explanation for the broadband bursty electrostatic noise observed within the electron foreshock. ¿ American Geophysical Union 1996