How can a solar storm black out a power grid, as in Quebec in 1989?

A solar storm can black out a power grid because a rapidly changing magnetic field around Earth drives electric current through long wires on the ground. Geomagnetically induced currents, or GICs, are electrical currents induced at the Earth's surface by rapid changes in the geomagnetic field caused by space weather. They can affect long conductors such as power transmission grids and buried pipelines.

The physics is old, Faraday's law of induction: a changing magnetic field induces an electric field at the surface, and that field pushes current through any grounded conducting structure, such as a power grid. Grids, pipelines, undersea cables, telephone and telegraph networks and railways are all long conductors of this kind. The effect was first seen on the telegraph network in 1847 and 1848.

The best-known case is Quebec on 13 March 1989. A severe geomagnetic storm collapsed the Hydro-Québec power grid in a matter of seconds, as protective equipment tripped one after another in a cascade. Six million people were left without power for nine hours. The utility's very long transmission lines, and the fact that most of Quebec sits on a large rock shield that kept current from flowing through the ground, sent the current along the 735 kV power lines instead.

The same night, the aurora was seen as far south as Texas and Florida, and, because it was the Cold War, some people worried that a nuclear first strike might be in progress. It was the Sun.