Ancient Trees and Modern Telescopes Hint at Our Sun’s Hidden Power
published during a new moon.
04/08/2016

 

ancient trees

A solar flare in action. Credit: NASA/GSFC/SDOAIA

An ancient Japanese Cedar with a secret

Hidden in the tree rings of an ancient Japanese Cedar is a story, recorded by no written word that we know of, of two devastating and almost certainly extraterrestrial events.

Each year as this mighty cedar grew, it took in carbon from the atmosphere and converted some of it to wood (as trees do).  But the rings that represent the years AD 775 and AD 994 are unique. The carbon in these rings is a bit more radioactive than would be expected for its age.

Under typical conditions, there is naturally a small but constantly replenished supply of radioactive carbon called carbon-14 in the atmosphere. After being taken in by trees and plants, its concentration declines over time due to radioactive decay — that’s how carbon dating works.

The obvious explanation for these anomalies is that there was a whole lot more radioactive carbon in the atmosphere during these particular years, but the more complicated question involves why. More than likely the spike was caused not by anything on Earth, but by a massive influx of protons from some source out in space. When these high-energy particles slam into the nitrogen of our Earth’s atmosphere, they convert some of it into carbon-14.

Though other theories such as nearby supernova explosions have been presented for these radioactive tree-ring spikes in the past, an increasing body of work suggests that the source is likely our own Sun. The unnerving consequence of this conclusion is that our sun must, therefore, be capable of producing solar flares bigger than anything we have ever directly recorded.

An ornery Sun

Solar flares are well-known events. The National Oceanic and Atmospheric Administration even has an office dedicated to monitoring their occurrence. They are caused, most scientists agree, by a breakdown in the Sun’s magnetic field, but the exact mechanism is complicated and not fully fleshed out.

What we do know is that when solar flares occur, they eject an energetic stream of particles which, traveling at nearly the speed of light, can pound into Earth’s atmosphere and magnetic field, causing not only beautiful auroras like the arctic’s Northern Lights but also problems for our satellites, communication networks, and electrical grids.

But according to estimates, the events recorded in the Japanese cedar must have been bigger than anything witnessed we’ve heard of. The largest recorded solar flare event, named the Carrington Event, occurred in 1859 and produced auroras so large that they could be seen as far south as Texas and Cuba. The AD 775 event, according to admittedly rough estimates, may have been a full order of magnitude larger.

So is there an upper limit to the power our Sun might unleash? How would scientists go about answering that somewhat terrifying question?

Bright flashes of faraway stars

There have been hints of large stellar blasts from other stars in the cosmos for years. In 2012, a team of researchers using data from the exoplanet-hunting Kepler Space Telescope (which monitors the light intensity of thousands of stars at one time) noticed that there were a number of stars that had experienced super bright flashes. Could these events could be like our own solar flares and, if so, does that suggest our Sun could unleash something as powerful?

Christoffer Karoff and his colleagues sought to address this question in a recent paper published in Nature Communications. “We can observe something on other sun-like stars that looks like solar flares,” he explained, adding that they were “hundreds to ten thousand times stronger than solar flares.” He said it is tempting to say that those events are simply solar flares scaled up, but there is no evidence to support the assertion either way.

To investigate that assumption, Karoff used data from the LAMOST telescope. This telescope — like Kepler — monitors thousands of stars at a single time but — unlike Kepler — monitors multiple spectra of visible and nonvisible light. Through complicated physics and some assumptions, this telescope allowed Karoff to measure the strength of the magnetic field of these stars over time.

ancient trees

The LAMOST Telescope, also known as the Guo Shoujing Telescope. Credit: Paul Hilscher

If, Karoff and his team reasoned, superflares on these other stars were associated with a weakening of that star’s magnetic field, then that would be evidence that the mechanism for both these crazy superflares and our Sun’s flares is the same.

Turns out that is exactly what’s been happening with these stars — the super flare events witnessed have all been associated with a weakening of that star’s magnetic field. This is some solid evidence that those tree ring events were caused by superflares similar to the mysterious flashes we have witnessed in space.

So we are all screwed, right? 

Luckily for us, most of those stars producing super flare events had much stronger magnetic fields that our own Sun. That means that it is extremely unlikely that our Sun could produce superflares comparable to those massive ones witnessed elsewhere in our galaxy.

But!

Around 10% of those stars Karoff and his team monitored had magnetic fields equal to or less powerful than our Sun’s. This lends even more credence to the idea that there have been much larger flares from our sun in the past, but it doesn’t mean we are going to be subject to a superflare as large as the largest ones witnessed by the Kepler telescope.

Any future event “would likely not be a full-scale super flare,” Karoff said, and added that “It would not compare to the largest superflares that we have observed with the Kepler mission.” Events on this scale would be quite destructive to satellites and electrical grids, and may also have the unfortunate effect of lowering the amount of protective ozone in our atmosphere, but at least they wouldn’t blow our atmosphere into oblivion as some of the larger events might.

Hugh Hudson, a Berkeley physicist and expert on solar flares, cautions that there are still a number of assumptions built into this study that will need to be addressed. The main concern, he said, is how the energy of these extrasolar superflares is calculated.

The calculations are based solely on the visible light and a direct comparison to our Sun, something called the “solar paradigm.” This doesn’t take into account other wavelengths of light or different mechanics of sunspots or other surface features on a star. It’s a “plausible but risky assumption to say that you can just scale directly from the solar paradigm,” he explained.

Telling the same story

Those assumptions aside, Karoff’s detailed astronomical study dovetails very nicely with the recent tree-ring research. His broad calculations based on studying those superflares suggest that these minor super flare events would occur roughly once a millenia — something that looks similar to the frequency of events seen in the trees. Hudson, for his part, has zero doubts that the tree ring events are from solar flares, calling that data “fantastic stuff” and an “eye-opener.”

These are the moments scientists live for — two completely different areas of research slowly approaching the same novel discoveries about the universe that we live in and the perils that they brings us.

 

Ancient Trees

An example of annual tree rings. Credit: Arnoldius