The largest solar flare in the last 500 years may have shredded Earth's ozone layer to a greater extent than human-made chemicals have in recent decades, new research suggests, but the effect was only temporary. If such a flare occurred today, it would likely be even more damaging to the ozone and could increase the rate of skin cancer around the world.

On 1 September 1859, the Sun expelled huge quantities of high-energy protons in a 'superflare'. The event was seen on Earth by an observer who noticed a white spot on the Sun suddenly brighten for about five minutes.

When the magnetic storm struck Earth, fires started in telegraph stations due to electrical arcing in the telegraph wires. The northern lights, or aurorae borealis, were reportedly seen as far south as Florida in the US.

This flare released 6.5 times more energy than the largest solar flare of the satellite era, which occurred in 1989. That flare was strong enough to cause a power blackout in Quebec, Canada.

Now, scientists have calculated the ozone depletion from the 1859 solar flare for the first time by studying chemical deposits in Greenland ice cores.

Acid rain

The deposits were laid down after the flare set off a series of reactions in Earth's atmosphere. For roughly two days after the flare, high-energy protons entered the atmosphere through the polar regions, channelled there by the planet's magnetic field lines.

The protons ionised nitrogen and oxygen molecules in the atmosphere, which then formed nitrogen oxides. The nitrogen oxides in turn reacted with ozone – a molecule made up of three oxygen atoms, breaking it into oxygen molecules and atomic oxygen.

This breakdown caused global atmospheric ozone levels to drop by 5%. In comparison, chlorofluorocarbons (CFCs) and other chemicals have depleted the levels by about 3% in recent years, says team member Adrian Melott, a physicist at the University of Kansas in Lawrence, US.

However, unlike CFCs and other ozone-depleting chemicals, which can persist in the atmosphere for some time, the flare-induced ozone thinning probably lasted for just four years, the researchers report. That is because the nitrogen oxides that cause the depletion eventually rain down with water or ice. Indeed, it was this acid rain that was eventually recorded in the ice cores.

Skin cancer

If such a superflare occurred today, it would likely have an even greater effect on the atmosphere, since the ozone layer is already depleted due to CFCs and other human-made chemicals.

"It certainly wouldn't be helped any," says team leader Brian Thomas, an atmospheric modeller at Washburn University in Topeka, Kansas, US.

Since the ozone layer normally blocks some of the Sun's dangerous ultraviolet radiation, a superflare would probably mean an increase in skin cancer due to the higher levels of UV light reaching the ground. The magnetic storm could also fry the electronics on many satellites and cause power outages on Earth.

Radiation sickness

Previously, another team used the ice core data to calculate that the radiation from a flare like the one in 1859 could cause acute radiation sickness – or even death – in astronauts shielded by the hull of an average spacecraft (see Superflares could kill unprotected astronauts).

The researchers say their work could have implications for hunting for life on other planets, since other Sun-like stars have been shown to have superflares. "It could be bad for critters that live there," Thomas told New Scientist.

It is not clear what causes the superflares on those stars. But if a planet larger than Jupiter is circling close to the star and causing the star's magnetic field to get twisted up, the resulting tension could be released in a massive solar flare (see Crisis, what crisis?).

Journal reference: Geophysical Research Letters (doi: 10.1029/2006GL029174, 2007)

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