Mysteries of the Carrington Event, the largest solar superstorm in modern times, unraveled by tree rings

HELSINKI, Finland — In the annals of astronomy, few events have captured the imagination quite like the Carrington Event of 1859. Named after the amateur astronomer Richard Carrington who first observed it, this solar superstorm sent shockwaves through the scientific community and left a lasting impact on our understanding of the Sun’s power. A March 2024 study by an international team of researchers sheds new light on this cosmic cataclysm, revealing unexpected clues hidden in the most unlikely of places: the rings of ancient trees.

To understand the significance of this discovery, we first need to take a step back and explore what makes the Carrington Event so special. On Sept. 1, 1859, Carrington was observing sunspots on the Sun’s surface when he witnessed an intense burst of white light – a solar flare. This flare was just the beginning of a cascade of events that would send a massive cloud of charged particles hurtling toward Earth, triggering what we now know as a geomagnetic storm.

The impact of this solar onslaught was felt around the globe. Auroras, typically confined to polar regions, were seen as far south as Cuba and Hawaii. Telegraph systems went haywire, with operators reporting receiving electric shocks and seeing sparks fly from their equipment. Some even claimed they could send messages without connecting their batteries, powered solely by the storm-induced currents surging through the wires.

In the 164 years since the Carrington Event, scientists have been piecing together the puzzle of what exactly happened and how it might help us prepare for future solar storms. Enter Dr. Joonas Uusitalo and his colleagues, whose recent study published in Geophysical Research Letters adds an intriguing new piece to this cosmic mystery.

Turning to radiocarbon for Carrington Event answers

The key to their breakthrough lies in a curious quirk of nature: the way trees record the Sun’s activity in their annual growth rings. When high-energy particles from solar flares or cosmic rays bombard Earth’s atmosphere, they can trigger a chain reaction that produces a rare form of carbon called carbon-14 (14C), or radiocarbon. This radioactive isotope gets absorbed by trees during photosynthesis, leaving a distinctive signature in the wood that persists for thousands of years.

“Radiocarbon is like a cosmic marker describing phenomena associated with Earth, the solar system and outer space,” says study co-author Markku Oinonen, Director of the University of Helsinki’s Laboratory of Chronology, in a media release.

By meticulously analyzing tree rings from around the time of the Carrington Event, Uusitalo’s team hoped to find a spike in radiocarbon that could provide a time-stamp of the solar storm. What they discovered, however, was far more surprising.

When comparing samples from trees in northern Finland to those from mid-latitudes, the researchers found a puzzling discrepancy. The high-latitude trees showed a significant excess of radiocarbon in the years immediately following the Carrington Event, while their mid-latitude counterparts showed no such increase. This “transient offset,” as the researchers termed it, lasted for several years before radiocarbon levels equalized.

A magnetic explanation

So, what could cause this geographic disparity? The answer, it seems, may lie in the complex interplay between the Sun, Earth’s magnetic field, and our planet’s atmosphere.

The trees of Lapland are a unique natural archive for investigating the past behaviour of the Sun. Markku Oinonen is drilling a sample containing interesting information on events in the 19th century.

One hypothesis is that the Carrington Event triggered a massive influx of radiocarbon-producing particles into the upper atmosphere over the poles – a region known as the stratosphere. From there, the radiocarbon-rich air slowly trickled down into the lower atmosphere, where it could be absorbed by trees. This process, known as stratosphere-troposphere exchange (STE), is thought to occur more rapidly at high latitudes, which could explain why the Finnish trees showed an earlier and more pronounced radiocarbon spike.

Intriguingly, this isn’t the first time such a latitude-dependent radiocarbon signal has been detected. Similar patterns were observed in tree rings dating back to two other massive solar storms in 774 and 993 CE. This suggests that the Carrington Event, for all its historical significance, may have been just one in a series of extreme solar events that left their mark on our planet’s carbon cycle.

But the story doesn’t end there. When Uusitalo and his colleagues tried to replicate their findings using state-of-the-art computer models of atmospheric radiocarbon transport, they hit a snag. The models couldn’t accurately reproduce the observed high-latitude radiocarbon excess, indicating that there may be gaps in our understanding of how radiocarbon moves through the atmosphere, especially in the polar regions.

NASA’s Solar Dynamics Observatory captured this image of a solar flare – as seen in the bright flash toward the middle of the image – on May 10, 2024. The image shows a subset of extreme ultraviolet light that highlights the extremely hot material in flares and which is colorized in gold.

This discrepancy highlights the importance of studying past solar storms not just for their own sake but for what they can teach us about the intricate dance between the Sun and Earth’s climate system. By unraveling the mysteries of the Carrington Event and its ilk, we may gain valuable insights into atmospheric circulation patterns, ozone chemistry, and the behavior of Earth’s magnetic field – all crucial pieces of the global climate puzzle.

Moreover, in an era when our technological infrastructure is increasingly vulnerable to the whims of space weather, understanding the frequency and severity of past solar storms could help us better prepare for future ones. Just as the Carrington Event wreaked havoc on the telegraph networks of the 19th century, a similar event today could potentially disrupt power grids, satellite communications, and GPS systems worldwide. Such is the case on May 10, when a massive geomagnetic storm caused radio blackouts across much of the eastern hemisphere.

“Ultimately, the observation emphasizes the role of subtle radiocarbon differences in tree-ring radiocarbon studies, potentially opening new ways to study past solar phenomena and atmospheric dynamics,” the authors write. “An improved understanding of the statistics of extreme solar events would be valuable when assessing the potential societal risks they pose. Furthermore, they may provide important insights into the multifaceted atmospheric dynamics and STE processes free of the contaminations of the modern era with anthropogenic radionuclide and fossil carbon signals.”

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