What Is Space Weather? Solar Storms and the Grid They Could Knock Out

The Sun is not a steady lamp. Its surface is threaded with magnetic fields that twist, tangle and snap, and the snapping releases energy in three forms that reach the Earth at three speeds. A solar flare is a burst of light across the spectrum, X-rays and ultraviolet included, that arrives in eight minutes and heats the upper atmosphere, disrupting the radio that bounces off it. The solar wind is the steady stream of charged particles that flows out at several hundred kilometres a second and takes days, gusting after a flare. And a coronal mass ejection is a billion tonnes of magnetised plasma thrown off the Sun at up to a few thousand kilometres a second, arriving in one to three days; if its magnetic field is aligned against the Earth's, the two connect and the storm is a big one. All three peak every eleven years with the sunspot cycle, whose current maximum is in 2024 and 2025.

The Earth's magnetic field deflects most of the solar wind, and the collision compresses the field on the day side and stretches it into a tail on the night side. A mass ejection that connects with it pours energy into the tail, which snaps back and drives particles down the field lines to the poles, where they light the auroras, and it sets the field itself swinging, which is the geomagnetic storm. The effects, in order of concern:

• •Auroras: the display, moving toward the equator as the storm grows, seen from Britain and the northern United States a few times a decade and from the tropics in the largest events
• •Radio and GPS: the disturbed ionosphere absorbs and scatters high-frequency radio, blacking out aircraft and shipping communications, and bends the signals from navigation satellites so that positions err by metres
• •Satellites: the heated upper atmosphere swells, dragging low satellites down, as it did to forty newly launched Starlink satellites in February 2022, and charged particles damage electronics and solar cells
• •Astronauts: the radiation from a large event is a serious dose outside the magnetosphere, and the crew of a Mars voyage would need shelter
• •The grid: a changing magnetic field induces currents in long conductors, and pipelines, railways and above all high-voltage transmission lines carry them into transformers that were never built for direct current, which overheat and can fail

The last item is the one that worries governments. In March 1989 a storm induced currents in Hydro-Quebec's grid that tripped its protection within ninety seconds and left six million people without power for nine hours in a Canadian winter; in 2003 a storm burned out transformers in South Africa. A Carrington-class event, ten times stronger, could damage large transformers across a continent, and large transformers are custom-built, weigh hundreds of tonnes and take a year or more to replace, so the estimates of the cost of a worst case run to trillions and the recovery to months. Grids have responded with monitoring, with procedures to reduce load and disconnect vulnerable transformers when a storm is forecast, and with the slow replacement of the most exposed equipment, and the question is whether the preparation is enough for an event that has not happened in the electrical age.

Space weather is forecast like the other kind, by watching the source. Satellites stare at the Sun in ultraviolet and X-rays and see the flare as it happens, coronagraphs see a mass ejection leave, and a spacecraft parked a million and a half kilometres sunward of the Earth, at the first Lagrange point, measures the plasma and its magnetic field as it passes, giving fifteen to sixty minutes of warning of what is about to hit. Forecasters in Boulder, Exeter and elsewhere issue watches for the days after a large ejection and warnings when the field is measured, on a scale from G1 to G5, and the forecasts have improved from guessing whether an ejection will hit at all to predicting its arrival within hours. What they cannot yet predict is the one thing that matters most, the orientation of the ejection's magnetic field, which decides whether a large storm is a light show or a disaster and is known only when the spacecraft feels it go by.

Storms of the 1989 class come every decade or two. The Carrington event is thought to be a once-in-a-century to once-in-five-centuries storm, and a near miss in July 2012, when an ejection of that size crossed the Earth's orbit a week after the Earth had passed, showed that the Sun still produces them; ice cores and tree rings record events in 774 and 993 AD that dwarf even Carrington, from causes on the Sun that are not understood. The Sun is quieter than most stars of its kind, which is one reason there is life here to worry about it, and the odds of a Carrington-class storm in any given decade are put at somewhere around one in ten, which for an event that could take down a continent's electricity is not a small number.

Space weather is the stream of light, particles and magnetised plasma from the Sun that arrives as flares, solar wind and coronal mass ejections and, when it connects with the Earth's magnetic field, drives geomagnetic storms that light the auroras, disrupt radio and GPS, drag down satellites and induce currents in power grids that can destroy transformers. The Carrington storm of 1859 set the benchmark, the 1989 storm blacked out Quebec, forecasters watch the Sun and a sentinel spacecraft for warning, and a century-class event in the electrical age remains the untested risk.