Rings of very old trees help Montana State physicist quantify strength of historic cosmic storms

BOZEMAN - Montanans have been dazzled in recent months by colorful displays of the aurora, or northern lights, which occur when plasma and energetic particles ejected by the sun slam into Earth's atmosphere and clash with the planet's magnetic field.

As spectacular as those light shows have been, Montana State University solar physicist Rachael Filwett says the solar energetic particle, or SEP, events that caused them were insignificant compared to others that have bombarded our planet in the distant past. Those events carried radiation levels dangerous enough to kill astronauts in space, wipe out satellites and space stations, and massively disrupt communication networks and other infrastructure on Earth.

"There's no comparison in modern space records," said Filwett, assistant professor in the Department of Physics in MSU's College of Letters and Science, who recently was awarded a three-year, $185,000 National Science Foundation grant to estimate the upper-limit strength of future extreme SEP events. "These are much larger than events like the recent flares that have caused the northern lights."

Though human instruments have never recorded an SEP event of such magnitude, scientists know that extreme high-energy particle storms have hit Earth's atmosphere at least three times over the last 2,400 years. The evidence is found where one might not expect: in the rings of very old trees.

In the atmosphere, SEPs from past storms interacted with nitrogen to form an abundance of carbon 14, which was then absorbed by trees through photosynthesis. By analyzing the growth rings of ancient trees, scientists have been able to measure spikes in levels of carbon 14 in the atmosphere and link them to specific dates.

Filwett will take data from three widely documented spikes that occurred in the years 663 B.C., A.D. 774 and A.D. 993 and use them in models to predict the timing, duration and maximum magnitude of future solar storms. But to ensure the accuracy of her final predictions, she said, more information is needed about how and at what rate different trees absorb atmospheric carbon.

"We're not treating these trees as a perfect measurement, because trees may hold onto carbon a little bit before absorbing it," Filwett said. "We're looking to understand the uncertainties in tree ring measurements and what they imply."

Amy Hessl, an expert in paleoclimatology from West Virginia University, and Mariah Carbone, a geophysicist from Northern Arizona University, are collaborating with Filwett to answer those questions. They will measure the amounts of carbon in the wood of living conifers, deciduous conifers and deciduous angiosperms growing in Utah, Missouri and North Carolina, then use those data to determine the differences in carbon-storage behavior between tree types. Carbone and Hessl then will compare their results with datasets from past extreme carbon 14 production events from the same sites, as well as with the global dataset of carbon 14 measurements, to determine how tree physiology and geomagnetic latitude affect carbon storage.

Filwett said the carbon uptake rates of trees aren't the only unknown factors that will be considered in the study. Though existing research suggests that the high carbon levels found in tree rings seem to line up with what would have been peaks in the sun's approximately 11-year solar cycle, some scientists believe the spikes may have resulted from a different astronomical phenomenon. Filwett said that's "not out of the question."

"In science, understanding uncertainties is really important," she said. "Are these events just bigger versions of the types of solar events we see now or totally different physical phenomena? And if these events are the true upper range of what the sun can produce, how often do they happen? We may be able to get better analogies by looking at other stars."

Scientists do know for sure that the sun can generate solar storms far stronger than any experienced by a living human. In 1859, the planet weathered the Carrington Event - the most intense geomagnetic storm in recorded history - named for the amateur astronomer who witnessed a solar flare on the sun's surface and realized it was responsible for the extraordinary events that followed 17 hours later and 93 million miles away on Earth. The storm lit the skies with spectacular auroral lightshows from the poles to the tropics and wreaked havoc on telegraph equipment. Were an event of that magnitude to occur today, experts believe it likely would result in massive and potentially devastating communication and electrical power disruptions.

As remarkable as the Carrington Event was, however, Filwett said it wasn't big enough to show up in tree ring records. That means the magnitude of SEP events she is working to quantify in her study - and for which humans need to prepare - is far, far greater.

"It's both really exciting and kind of terrifying," she said.