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The sunspot chronicles

Natural History, Feb, 2004 by Joomi Kim

All those splendid auroras streaking across the sky at mid to high latitudes lately signal that the Sun has been magnetically hyperactive, breaking out in a bad case of sunspots [see "Our Stormy Sun," by Charles Liu, page 64]. But how often does the Sun's magnetism get in an uproar?

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Now Ilya G. Usoskin, a geophysicist at the University of Oulu in Finland, and colleagues from the Max Planck Institute for Aeronomy in Katlenburg-Lindau, Germany, have discovered that the past sixty years have been the Sun's most magnetically active period in more than a millennium. Almost as intriguing as the discovery itself is the method that led to it: constructing a sunspot record by examining the amounts of a radioactive isotope of beryllium measured in ice cores from Greenland and Antarctica. The isotope, beryllium-10 (Be-10), is produced when cosmic rays--predominantly protons--collide with nitrogen and oxygen nuclei in the Earth's atmosphere. When the Sun is magnetically active, the "wind" of charged particles emanating from it (and dragging along its magnetic field) increases, deflecting cosmic rays and so cutting down on the production of Be-10. The concentration of Be-10 at a given, independently dated level in an ice core reflects the intensity of magnetic activity in the Sun.

Before the team's analysis, the only reliable records of solar magnetic activity were direct counts of sunspots, and those weren't made until 1610, soon after the invention of the telescope. The Be-10 data extend the "fossil record" of the Sun's past activities back to A.D. 850, enabling solar physicists to draw more reliable conclusions about long-term solar cycles. ("Millennium-scale sunspot number reconstruction: Evidence for an unusually active Sun since the 1940s," Physical Review Letters 91:1-4, November 21, 2003)

The sunspot chronicles

Natural History, Feb, 2004 by Joomi Kim

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