  A method to study electromagnetic (EM) effects generated by a straight null
cosmic string moving in classical EM fields is suggested. The string is shown
to induce an additional EM field which can be described as a solution to
homogeneous Maxwell equations with initial data set on a null surface, the
string event horizon, where the string world-sheet belongs to. The initial data
ensure the required holonomy of the string space-time caused by the gravity of
the string. This characteristic initial value problem is used to study
interaction of plane waves with null strings and perturbations by the strings
of the Coulomb fields of electric charges. It is shown that parts of an
incoming EM wave crossing the string horizon from different sides of the string
are refracted with respect to each other and leave behind the string a
wedge-like region of interference. A null string moving near an electric charge
results in two effects: it creates a self-force of the charge and induces a
pulse of EM radiation traveling away from the charge in the direction close to
trajectory of the string.
