Doomsayers are fond of citing a 1859 solar storm dubbed the Carrington event. An unusually massive coronal mass ejection (CME), a fast-traveling front of gas and magnetic field born among the most active regions of the Sun's surface, ripped through space and slammed into Earth's magnetosphere, compressing it to an unusually extreme degree. The resulting shock waves induced powerful geomagnetic storms. Auroras could be seen as far south as Cuba, while telegraph lines across the US caught fire and exploded; in some cases, operators were even subject to electric shocks.
This is of course classic fodder for apocalyptic cable shows and the effects of a sub-one-in-a-lifetime solar storm are well-advertised and not infrequently exaggerated. The Sun has a unique way of making us feel vulnerable like few other things because, well, it's absolutely everything to us and our survival. There's no fix for a malfunctioning Sun, real or imagined. Solar activity is probably the purest Act of Nature we know.
The good news is that, according to research presented at the International Astronomical Union General Assembly last week, we're probably still a ways off from the next event of this magnitude. This is often referred to as a superflare, though it may be more accurately referred to as just a hell of a solar flare. The nomenclature is a bit muddy, and many astronomers argue that the Solar System has likely never seen a true superflare, let alone in recent history, as it would have been strong enough to strip away our entire ozone layer, blanket the planet is harsh radiation, and basically ruin human civilization outright.
And yet here we are, while even pre-human history offers little evidence for superflare occurrences.
Predicting an epic solar flare this is tricky business, given the simple fact that we've only really observed a single superflare-ish event (in 1859), which is where our solar activity data begins. The first definite, clear CME detection only came in 1971.
The astronomers behind the current research—a team based at the Harvard-Smithsonian Center for Astrophysics—used data collected from 84 Sun-like stars over a four year period, observing a total of 19 superflares. Based on this, they were able to come up with a probable superflare frequency among stars like our own: It should happen only once every 250 to 480 years, with the most likely scenario being once every 350 years.
The Harvard-Smithsonian presentation was first reported in a Science News hit, but I haven't been able to find the actual source online. Meanwhile, the Daily Mail and a bunch of similar sites have derived their own hits from Science's single paragraph. I'll just assume then that all of this has to do with data collected in recent years by the Kepler Space Telescope, which has been stalking Sun-like stars for sunspot/superflare data. The past couple of years have seen a lot of studies on estimated superflare frequencies.
Astrophysicists at Kyoto University presented some Kepler-derived research in May that tells a somewhat different story, for example, estimating that a superflare should occur on Earth every 10,000 years. A 2012 study based on Kepler data was a bit less optimistic, suggesting a frequency of up to once every 800 years. Both are notable at least for the suggestion that our Sun can produce a superflare at all; the phenomenon is usually reserved for much younger and more energetic stars. The Carrington event should probably be more accurately referred to as just an unusually extreme CME rather than a superflare, which would have done a lot more damage than frying some telegraph wires, including the aforementioned Armageddon.
I suspect the Harvard-Smithsonian presentation is talking about Carrington-class flares rather than superflares. Superflares should be highly unusual among stars like our own Sun—superflare-producing stars tend to not just rotate much faster, but have much more sunspot activity. Let's hope so, anyway.