SUPERFLARES: Earth under threat
The risk to our planet from coronal mass ejections may have been seriously underestimated, as Marcus Chown discovers
The energy of the Sun powers all living things on Earth, alongside all the advances in human technological civilisation. But what the Sun has so generously given us it could take away in an instant. According to two astronomers in the US, a ‘superflare’ could send us back to the pre-electrical age or, at worst, trigger a global mass extinction.
Manasvi Lingam and Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics say much attention has been paid by scientists and governments to the threat of an asteroid impact on Earth but very little to the threat posed by a violent eruption from the Sun. And they believe the latter is at least as serious a threat as the former.
What has brought this threat into sharp focus is the observation of around 100,000 Sun-like stars by NASA’s Kepler satellite in Earth orbit. To everyone’s surprise, such stars are not as stable as previously thought but instead prone to extraordinarily violent outbursts, and the obvious inference is that the Sun may be no different.
If the Sun were merely a hot ball of gas, it would be dull and boring, its behaviour easy to understand and predict. In fact, in the 1920s, the venerable English astronomer Sir Arthur Eddington was able to devise a theory of what was going on in the solar interior despite having no knowledge of the nuclear reactions that are the Sun’s ultimate power source.
The Sun, however, is not merely a hot ball of gas: what changes everything – and makes it capable of launching deadly solar flares into space – is the presence of its magnetic field.
The Sun of all fears
The Sun’s magnetic field, which is not unlike that of a bar magnet, is generated by electrically charged currents of matter that are constantly churning deep within its interior. Because the Sun is not a solid body, its interior rotates at a different rate to its surface and its rotation speed also varies with latitude. This causes the solar magnetic field ‘lines’ to wind up, storing up energy like twisted rubber bands.
When the tension becomes too great, these field lines ‘reconnect’ to a lower-energy state. High-energy protons and electrons, which were formerly confined by the field, are suddenly
set free and violently catapulted off into space. This is what is known a solar flare.
The biggest flare in the age of science was recorded by English amateur astronomer Richard Carrington on 1 September 1859. Now dubbed a ‘coronal mass ejection’, the ‘Carrington event’ involved the Sun ejecting solar material comparable to the mass of Mount Everest at about 1,000 times the speed of a passenger jet. When the magnetic field it carried reached the Earth, it sliced through electrical conductors, inducing currents so massive they were enough to electrocute telegraph operators.
What is worrying about Kepler’s observations is that they reveal flares on solar, or G-type, stars up to 20,000 times bigger than the Carrington event. Fortunately for us, such enormous flares appear to occur only every 20 million years or so on any given star. However, Lingam and Loeb point out that there is some evidence of a 26 million-year periodicity in terrestrial extinction events. They speculate that, if this is real, it could be explained by superflares.
A flare essentially creates high-velocity protons and high-energy ultraviolet light. Both have the ability to destroy the ozone shield that protects life on the surface of the Earth from deadly solar ultraviolet radiation. Not only can such ultraviolet directly damage the DNA of organisms but it can reduce the ability of phytoplankton to photosynthesise, a process on which life in the oceans depends. As such organisms decline, so too does their ability to suck the greenhouse gas carbon dioxide from the atmosphere, resulting in rising
A dramatic visual of a coronal mass ejection (CME) composited from two images captured by the Solar and Heliospheric Observatory (SOHO) – one of the Sun, the other of the CME – taken at roughly the same time in January 2002
A sketch of the sunspots seen by Richard Carrington on 1 September 1859. He watched as the points of activity marked A and B moved to points C and D over the course of five minutes, then vanished