Analyzing polar ice to track solar activity.

Analyzing polar ice to track solar activity

Scientists have a complete, consistent record of solar activity--expressed as the number of sunspots--going back to 1843. Clues to solar activity in even earlier periods come from historical documents containing references to auroras and sunspots. However, despite concerted efforts to extend the sunspot record further, data from the period before 1600 remain incomplete and uncertain.

Researchers in Switzerland have now demonstrated that the levels of the radioactive isotope beryllium-10 in samples of polar ice follow variations in solar activity. Peaks in isotope concentration correlate well with solar minima, when few sunspots appear, providing a way of tracking the well-known, 11-year solar cycle over a longer time span.

"Our study proves that [beryllium-10] in polar ice is a useful tool to extend significantly the historical record of solar activity," the team reports in the Sept. 13 NATURE. "This opens up the possibility of studying the long-term behaviour of solar activity and the history of solarterrestrial relationships." Such studies could provide insights into how solar activity influences climate and what processes drive the solar cycle.

Scientists have known for several decades that solar activity appears to modulate the flux of cosmic rays reaching Earth's atmosphere. Magnetic fields in the solar wind emanating from the sun apparently deflect low-energy cosmic rays, especially during periods of intense solar activity, and that shift depresses the normal rate of production of radioactive isotopes in the atmosphere by as much as 25 percent.

The most promising isotope for tracking solar activity is beryllium-10, the researchers say. Produced by cosmic-ray-induced nuclear reactions in the atmosphere, beryllium-10 initially clings to particles floating in the air. Precipitation flushes the beryllium-laden particles out of the air, depositing them on Earth's surface within a year or two of their appearance.

To test whether beryllium-10 might be useful for tracking solar activity, the researchers analyzed part of an ice core from Greenland and compared the measured beryllium-10 concentrations with sunspot records from 1783 to 1985. Despite short-term fluctuations in beryllium-10 concentrations due to changes in precipitation patterns and other atmospheric effects, they could easily pick out an 11-year cycle and other, longer-term trends in the beryllium data that closely match the sunspot record.