  There is a growing concern that a big coronal mass ejection event will induce
perturbations on the power supply of fiber optic transoceanic cables that may
produce a global internet blackout. In this paper we give the expression of the
voltage variations that a transient change of the geomagnetic field induces on
the voltage of the power supply of a transoceanic fiber optic cable. We show
that the transient voltage change is proportional to the magnitude of the
magnetic field deviations and not to its time derivative as a direct
application of Faraday's law would imply, and this suggests design criteria to
protect transoceanic fiber optic systems against big geomagnetic storm events.
The presented analysis also enables the classification of existing systems into
some that are less sensitive to the weakening of the geomagnetic field
occurring during strong geomagnetic storms and others that are more prone to
experience an outage when a weakening of the geomagnetic field occurs.
