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 solar 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 solar storms and others that are more prone to
experience an outage when a weakening of the geomagnetic field occurs.