The Carrington Event: The 1859 Solar Storm That Set Telegraph Wires on Fire

A brewer, an astronomer, and a very bright flare

On the morning of 1 September 1859, the English amateur astronomer Richard Carrington was sketching sunspots at his private observatory near Redhill, south of London, when part of a large sunspot group suddenly brightened into two intensely white points of light. The flash lasted only a few minutes, but Carrington had just become the first person to witness a solar flare and correctly connect it to a solar origin. He could not have known it at the time, but he was watching the opening act of the most intense geomagnetic storm in recorded history.

Less than a day later, Earth was struck by the associated eruption of solar plasma - what we now call a coronal mass ejection, or CME. Typical CMEs take three to five days to cross the 150 million kilometres between the Sun and Earth. This one, cleared of resistance by a smaller eruption that had preceded it and travelling at an estimated 2,000-3,000 kilometres per second, arrived in roughly 17 hours.

A night the sky caught fire

When the CME's magnetic field connected with Earth's, the resulting geomagnetic storm produced auroras of an intensity essentially never witnessed since. Reports describe skies bright enough to read a newspaper by at local midnight, and aurora seen far outside its usual polar range - observers in the Caribbean, Colombia and Hawaii reported red glows low on the horizon, and even people at tropical latitudes saw colour in the sky. Gold miners in the Rocky Mountains reportedly got up and began preparing breakfast, convinced dawn had broken.

The technological damage fell on the one continent-spanning electrical network that existed at the time: the telegraph. Induced currents in the long telegraph wires were strong enough to shock operators, ignite paper at some stations, and in a few well-documented cases allow operators to send and receive messages after disconnecting their batteries entirely - the geomagnetically induced current alone was powering the line.

How big, really, in modern units

Because dedicated geomagnetic instruments were still primitive in 1859, the storm's exact strength has to be reconstructed from magnetometer traces, auroral latitude and ice-core nitrate records. The best modern estimates put it at the very top of NOAA's storm scale - G5, "Extreme" - with a planetary Kp index that would have pegged the standard 0-9 scale, and a Dst index (a measure of the ring current that intensifies during storms) plausibly beyond -1,600 nanotesla. For comparison, a strong modern storm might reach -300 to -400 nT.

Two more recent storms give a useful sense of scale. The March 1989 storm that collapsed Hydro-Québec's grid within about 90 seconds, cutting power to six million people for roughly nine hours, is thought to have been perhaps a tenth as intense as the Carrington event. The 2003 "Halloween storms" caused a blackout in southern Sweden and damaged several satellites, and were still well short of 1859 levels.

What a repeat would look like today

The difference between 1859 and today is not the Sun - it is us. In 1859 the only continent-scale electrical infrastructure was the telegraph network, and the damage, while dramatic, was recoverable within days. A Carrington-class storm striking a modern grid would induce currents in transformers across high-voltage transmission networks at high and mid-latitudes, with the risk of transformer damage that cannot be repaired quickly because large power transformers are custom-built and typically take months to manufacture and ship.

Widely cited assessments - including a 2013 Lloyd's of London study and a 2008 US National Academy of Sciences report - put the potential cost of a modern Carrington-scale event at anywhere from roughly a trillion to several trillion pounds, with recovery measured in years rather than weeks, driven mainly by extended, cascading power outages. GPS, satellite communications, and aviation on polar routes would all be affected to varying degrees, though - importantly - none of this poses a direct physical danger to people; geomagnetic storms do not harm human bodies directly.

In the UK, National Grid and Ofgem have both studied geomagnetically induced current risk specifically because Britain sits at a latitude, and has a grid geometry, that makes it more exposed than most of continental Europe. Since the mid-2010s the UK has treated severe space weather as a formal entry on its National Risk Register.

Could it happen again?

Almost certainly, yes - the question researchers ask is "when", not "if". Statistical estimates based on the historical and ice-core record suggest Carrington-class events occur roughly once every one to two centuries, which implies something like a 1-in-10 chance in any given decade, though these are necessarily rough figures given how few such events have ever been measured directly.

We came close in July 2012, when a CME of comparable speed and size to the Carrington event erupted from the Sun but missed Earth, having launched from a region of the Sun that had rotated away from our planet roughly nine days earlier. Had the timing been different, the resulting storm would likely have rivalled or exceeded 1859. Spacecraft at the L1 point between the Sun and Earth, such as NASA's ACE and NOAA's DSCOVR, now give forecasters roughly 15 to 60 minutes of precise warning once a CME's leading edge is detected, plus one to three days of general warning once an eruption is observed on the Sun - useful, but far from the months of notice we would ideally want for something this consequential.