  We study relativistically expanding electromagnetic fields of cylindrical
geometry. The fields emerge from the side surface of a cylinder and are
invariant under translations parallel to the axis of the cylinder. The
expansion velocity is in the radial direction and is parametrized by
$v=R/(ct)$. We consider force-free magnetic fields by setting the total force
the electromagnetic field exerts on the charges and the currents equal to zero.
Analytical and semi-analytical separable solutions are found for the
relativistic problem. In the non-relativistic limit the mathematical form of
the equations is similar to equations that have already been studied in static
systems of the same geometry.
