Our Sun could be overdue for a violent super-eruption, study warns

Our Sun is not a peaceful place. It bubbles by convection; its magnetic field breaks, finds a connection, closes again. It triggers eruptions of energy in the form of violent flares, and of plasma in the form of coronal mass ejections.

Most of this activity isn’t powerful enough to harm us…but every once in a while the Sun erupts with a flare powerful enough to cause serious damage. And we don’t know how often such events occur. Previous estimates ranged from once per century to once per millennium.

Well, folks, we have a problem. Because a new analysis of the flare rates of 56,400 Sun-like stars has estimated that the Sun’s super-flare rate is at the lower end of that scale – once every 100 years. If so, we could be in trouble, because even the infamous Carrington event, which took place in September 1859, was only 1% as powerful as a super-eruption.

“We were very surprised,” says astronomer Valeriy Vasilyev of the Max Planck Institute for Solar System Research in Germany, “that Sun-like stars are subject to such frequent superflares.”

Determining how often the Sun emits a giant emission of radiation is not easy. We can’t exactly hit the rewind button on reruns. There are records of solar activity in tree rings that give us an idea (the largest storms driven by the Sun create a spike in carbon-14) and nitrogen in polar ice, but these might not not give us the full picture.

By searching for stars like our Sun – G-type yellow dwarfs – and hoping to catch some of them heating up, researchers could estimate the frequency of large-scale events. There’s just one problem: We can’t always easily measure the rotation rates of these stars, and because rotation can be linked to flare activity, the information we get from them is incomplete.

Vasiliev and his colleagues set out on their search for stars with two observations in mind. Sun-like stars with measurable rotation tend to be more active than the Sun. And the most Sun-like stars have rotation periods that are difficult to measure.

They decided to exploit these two facts to access a large sample of Sun-like stars, including stars with unknown rotation rates, but other characteristics as similar as possible to those of the Sun, namely the brightness and temperature.

They also excluded Sun-like stars with rotation periods less than 20 days (the Sun’s rotation period is 25 days). Indeed, the rotation of stars gradually slows down as the Sun ages; younger stars therefore have faster rotation rates. And young stars are more active than older stars of the same genus.

They managed to obtain a sample of 56,450 Sun-like stars and observed 2,889 superflares out of 2,527 of them. This equates to a super-eruption rate of about once every 100 years.

So what’s wrong with the Sun? Well, we still don’t know. We know this can cause epic tantrums. The Carrington event included both a solar flare and a coronal mass ejection that generated a powerful storm in the Earth’s magnetic field; it was the coronal mass ejection that caused the most damage.

Indeed, coronal mass ejections can generate currents which then travel through the ground and interfere with infrastructures and overload them. The Carrington event wiped out telegraph systems around the world, with some overloaded networks starting fires. There was also a major geomagnetic storm in 1989 that affected several power grids and caused power outages.

Scientists have discovered nine geomagnetic storm events stronger than the Carrington event in tree rings over the past 15,000 years, known as Miyake events. The most recent we have found is 774 CE. Miyake events are estimated to occur approximately every 1,000 years. But a coronal mass ejection does not accompany every flare emitted by the Sun.

“It is not entirely clear whether gigantic flares are always accompanied by coronal mass ejections and what the relationship is between superflares and extreme solar particle events,” explains astrophysicist Ilya Usoskin from the University of Oulu in Finland. “This requires further investigation.”

Solar flares are not without effects; they can temporarily disrupt high-frequency radio communications by changing the density of the ionosphere through which radio waves refract. However, given that the largest geomagnetic storms on record included both a solar flare and a coronal mass ejection, it is reasonable to be concerned about possible solar superflare activity.

Since the most effective defense against a giant geomagnetic storm is an accurate forecast, research suggests we need to better understand how our Sun works.

“These new data are a stark reminder that even the most extreme solar events are part of the Sun’s natural repertoire,” says astrophysicist Natalie Krivova of the Max Planck Institute for Solar System Research.

The results were published in Science.

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