The Tree of (Solar) Life

At the center of our solar system sits a hot ball of burning gas that provides the major source of energy for life on our planet. Sunspots offer us a precious glimpse at the level of solar activity beyond its surface, but our eyes have yielded little information over the past four centuries – and detailed, direct satellite observations of these temporary phenomena are limited to the last 70 years. Now, however, a team from the Laboratory of Ion Beam Physics at ETH has managed to look as far back as the last millennium by measuring radioactive carbon levels in tree rings.

”We analyzed nine different trees that had been dated by dendrochronology – using their growth rings as an indicator – at annual resolution with accelerator MS (AMS),” says Nicolas Brehm, lead author of the paper. “Using this technique, we were able to get the ¹⁴C/¹²C ratio of each year,” he adds. Any change in ¹⁴C levels would reflect a change in the level of protection offered by the Sun – which usually guards the Earth from radioactive cosmic particles through its magnetic field.

“The analysis of the 11-year solar cycle before the observation of sunspots is of particular interest because, until now, we have been limited to analyzing the amplitude and length of just 25 cycles – since precise measurements are only available from about 1750 onwards,” says Brehm. “We’ve shown that amplitudes – how much the sun’s activity fluctuates – of these cycles are significantly reduced during grand solar minima (periods of low activity) compared to maxima (high activity).”

Not only does this work provide new insight into solar dynamics, but their high-temporal-resolution record of atmospheric carbon provides a whole new data set for improving radiocarbon calibration procedures. “Our next steps are to extend the record further back in time to reconstruct solar activity in annual resolution and search for more energetic particle events,” adds Brehm.