The Sun suddenly became brighter, telegraph wires caught fire, and the aurora over Rome was bright enough to read a newspaper at night - it was the "Carrington episode" of 1859, the most powerful solar storm ever recorded.
It was shortly after 11 a.m. on September 1, 1859, when Richard Carrington, a prominent English astronomer, noticed something strange in his private observatory. As every day, his telescope projected the image of the sun's disk onto a white surface.
That morning, the sunspots were unusually numerous. Suddenly, two arcs of blinding light appeared above the spots, growing larger and taking the shape of a loop.
Sunspots drawn by Richard Carrington on September 1, 1859
"What Carrington saw was a solar flare of white light—a magnetic explosion on the Sun," explains David Hathaway, an astrophysicist at NASA's Marshall Space Flight Center in Alabama.
"It is rare to observe such an increase in the brightness of the solar surface," he points out.
Solar flares, which are caused by local disturbances in the solar magnetic field, typically emit high-energy X-rays and radio waves. Often, as was the case with the Carrington event, they are accompanied by so-called coronal mass ejections, or CMEs: billions of tons of particles are ejected into space.
This particular ejection headed directly toward Earth and caused the strange phenomena of the next day. Shortly before sunrise, the sky over almost the entire Earth was lit up by a red, green, and blue aurora, visible even in the tropics, from Cuba and Jamaica to El Salvador and Hawaii.
The spectacle was impressive, but it was accompanied by problems in the telegraph systems. Telegraph operators were electrocuted, telegraph paper caught fire. Even when the batteries that powered the lines were disconnected, the induced currents created by the aurora continued to allow messages to be transmitted.
According to NASA's David Hathaway, the 1859 event was the largest geomagnetic storm in 160 years of observations. Even if one looks for traces of such events in ice cores from the Arctic or Antarctic, the Carrington flare remains the strongest in at least 500 years.
Even milder solar storms can cause problems, however. In 1972, the American telecommunications company AT&T was forced to change the design of its transatlantic cables when telephone communications were disrupted by a geomagnetic storm.
Another flare on March 14, 1989 knocked out a hydroelectric plant in Quebec and left about 6 million people without power for 9 hours.
In December 2005, X-rays from another flare disrupted satellite communications and the GPS network. The outage lasted only ten minutes, but that could have been enough to throw planes off course.
Solar flares become more frequent and powerful at the so-called solar maximum, the peak of an 11-year cycle of solar activity.
The next maximum is estimated to come in the fall of 2013, but no one knows when an eruption similar to that of 1859 will occur again.
But what is certain is that next time we won't just be worrying about the telegraph. With humanity increasingly relying on electronic systems, damage to power grids, mobile phone networks and satellites could cost tens of billions of dollars, while the crew of the International Space Station will be at risk from the Sun's wrath.
The only positive thing is that this time we will have warnings from the spacecraft -SOHO, Hinode, SDO and others- that remain permanently pointed at the Sun.
