The Magnetic Surge

The Carrington Event and Geomagnetically Induced Grid Collapse

The telegraph paper tape recorded between Boston operator Thomas Bullard and his counterpart in Portland, Maine, on the morning of September 2, 1859 logged two hours of commercial dispatches transmitted across unplugged copper lines. Both stations had completely disconnected their acid batteries from the circuit. Across North America and Europe that morning, telegraph armatures sparked violently, set paper tape ablaze, and shocked operators who touched their brass keys. When the Boston operator proposed unplugging their chemical battery banks to see if the surging terrestrial current could carry the Morse signal on its own, the Portland station tapped back that they were operating on the auroral current alone and receiving clearer transmissions than when their batteries were connected.

Seventeen hours earlier, at 11:18 a.m. on September 1, English brewer and amateur astronomer Richard Carrington had been projecting an image of the Sun onto a painted glass screen inside his private observatory in Redhill, Surrey. As he sketched a sprawling cluster of dark sunspots, two kidney-shaped patches of blinding white light flared inside the magnetic tangle. Carrington had observed the first recorded white-light solar flare and the launch of an ultra-fast Coronal Mass Ejection (CME). That billion-ton cloud of magnetized solar plasma hurtled across interplanetary space at more than 2,000 kilometers per second instead of the usual 400 kilometers per second.

When that magnetized plasma cloud slammed into Earth's magnetosphere, it compressed the dayside magnetic field from sixty thousand kilometers altitude down below geostationary orbit. It drove million-ampere electric currents—ionospheric electrojets—hundreds of kilometers overhead. By Faraday's Law of Electromagnetic Induction, any rapid time-variation in a magnetic field $\mathbf{B}$ generates a curling electric field $\mathbf{E}$ across the conductive surface of the planet below:

In 1859, the only long electrical conductors spanning the continents were iron and copper telegraph wires. One hundred and thirty years later, on March 13, 1989, a solar storm less than one-third as intense as the Carrington Event struck a planet wired with high-voltage alternating-current power grids. At 2:44 a.m. Eastern Time, magnetic field variations over northern Canada reached nearly 500 nanoteslas per minute.

What turned that magnetic swing into an engineering disaster in Québec was the geology beneath the power lines. The Hydro-Québec grid transmits electricity across the Canadian Shield, a vast craton of ancient igneous granite and metamorphic gneiss with high electrical resistivity. When a changing magnetic field induces a horizontal geoelectric potential of several volts per kilometer across the Earth's surface, the current seeks the path of least resistance. Blocked by the resistive granite bedrock below, hundreds of amperes of quasi-direct geomagnetically induced current (GIC) flowed directly up the grounding rods into the neutral windings of Hydro-Québec's 735-kilovolt step-up transformers. From there the current raced down hundreds of kilometers of aluminum transmission lines.

Alternating-current transformers rely on an iron core whose magnetic flux swings symmetrically between positive and negative peaks sixty times a second. Even a small direct current injected into the winding biases that loop and drives the iron core into half-cycle magnetic saturation. Once saturated, the transformer core spills stray magnetic flux into its steel structural tank. It heats up by hundreds of degrees within seconds and floods the grid with harmonic distortions while gulping massive quantities of reactive power. Five transmission lines from James Bay tripped offline in succession. The entire Hydro-Québec power grid collapsed in ninety-two seconds, plunging six million people into darkness. After the 1989 blackout, Hydro-Québec spent over one billion dollars installing series capacitors on its long transmission lines to block quasi-direct geomagnetically induced currents before they reach transformer cores.