Aurora returns for an encore

The strongest geomagnetic storm of the current solar cycle hit the atmosphere on the night of 18 October, and the aurora it produced was visible from latitudes that have not seen the lights in a generation, from the northern states to the Mediterranean coast, with the photographs arriving on the morning feeds from people who had never photographed the sky before. The storm had been forecast three days in advance, with the kind of precision that the space-weather services have spent two decades building, and the forecast held: the arrival time, the intensity, and the southern reach of the display all matched the prediction. The science of the storm was a study in prediction working as intended, and the spectacle was a reminder of what the planet’s magnetic field does for free.

The source of the display was the same machinery that produced the spring’s big show, a sunspot region rotating back into view of the Earth with its magnetic complexity intact, and the coronal mass ejections it released arrived as a pair, compressing the magnetosphere and driving the auroral oval toward the equator. The forecasters’ confidence was earned by the satellites that watch the sun and the spacecraft that sample the solar wind an hour ahead of the Earth, the early-warning system that gives the grid operators time to protect the transformers and the airlines time to reroute the polar flights. The storm that produced the beauty was, on the technical ledger, a moderate event handled by a system designed for worse, and the handling is the part of the story that the photographs cannot show.

The difference between the spring display and this one is worth a paragraph, because it is the difference between the aurora as novelty and the aurora as science. The spring storm was the surprise, the event that exceeded the forecasts and caught the world’s attention. The autumn storm was the confirmation, the event that behaved as predicted and proved the forecasters right, and the two together form the arc of a maturing field: first the anomaly, then the norm. The sky-watchers who chased the spring display had the autumn encore to compare it against, and the comparison, by the accounts posted from the viewing sites, was close: the colours differed, the structure differed, and the wonder did not.

The storm’s significance beyond the spectacle is the quiet part. The same magnetic activity that painted the sky is the activity that the power industry monitors, that the satellite operators insure against, that the GPS systems correct for, and the October storm tested all three with no significant failures, which is the progress that the coverage of the lights rarely mentions. The infrastructure that the modern world depends on was designed, over two decades, to survive the events that the sun can throw, and the design was tested this week with the aurora as the visible proof. The lights themselves cost nothing and mean nothing to the grids; the prediction that preceded them is the expensive and invisible part, and it worked.

The aurora’s encore has now closed the show that the spring opened, and the sky-watchers are already checking the sunspot forecasts for the winter’s possibilities, because the cycle has not yet peaked and the good displays are likely not finished. The photographs from the October night will circulate for years, the way the spring’s did, and the science will move on to the next event, the next forecast, the next quiet test of the infrastructure that the lights advertise. The planet’s magnetic field does the work for free; the forecasters charge for the warning; and the night sky, as usual, provides the proof.