[ccebook.cn]The World in 2010

Science
Where have all the sunspots gone?
Nov 13th 2009
The sun will surprise scientists
Something odd has happened to the sun. Four centuries after sunspots were first
seen—by Galileo—they have disappeared almost entirely. In 2009 weeks and
sometimes months went by without a single sunspot being discerned. In 2010 they
will return, or so say most solar scientists. Others wonder whether the sun may be
going through an extended period of inactivity.
Sunspots are a bit of a mystery. They are transient tangles in the sun’s magnetic
field that are slightly cooler than their surroundings and so appear as dark patches
in the photosphere—the surface layer of the sun. They tend to appear in pairs (on
opposite sides of the sun) that persist for a fortnight or so before fading.
Normally the number of sunspots peaks every 11 years, coinciding with the times
when the sun’s magnetic field is at its strongest. As the field wanes, the number of
sunspots falls to a trough or minimum, at which point the sun’s magnetic field
reverses direction and starts to regain its strength. As it does so, sunspots begin to
appear close to the poles of the sun. When the magnetic field is at its strongest, and sunspots at their most
plentiful, they cluster close to the equator.
Back in 2008 solar scientists saw a high-latitude, reversed-polarity sunspot, suggesting the start of a new
solar cycle. Since then, however, there has been little activity. Reliable predictions of sunspot numbers are
impossible to make until the solar cycle is well established, usually three years after the minimum. So with
luck the sunspot riddle should be solved in 2010.
Scientists from America’s space agency, NASA, reckon that the next peak will come in March or April 2013.
However, their colleagues at the National Solar Observatory in Tucson, Arizona, have found that the
magnetism of sunspots (the strength of the knot that they form) has been declining over the past couple of
decades. If this carries on, solar magnetic fields will become too weak to form sunspots, which will vanish
completely in 2015.
In the past, sunspots have disappeared for decades. Between 1645 and 1715, they were rare. There were
several years in which none at all was sighted and others in which fewer than ten were spotted. Giovanni
Cassini, an Italian astronomer, described a sunspot that appeared in 1671 as the first he had seen for many
years. John Flamsteed, the first Astronomer Royal, observed one in 1684, after a gap of ten years. This 70year
period of low solar activity has since been dubbed the Maunder Minimum (after an astronomer called
Edward Maunder, a sunspot specialist).
Whether the sun has taken that particular path or will return to normal will become clear in 2010. Not only
astronomers will be interested in the answer. The Maunder Minimum coincided with a period of exceptionally
cold winters in Europe and North America and, perhaps, elsewhere. What happens if global warming meets
solar cooling? Expect a hot debate.
Alison Goddard: science correspondent, The Economist
Copyright © 2009 The Economist Newspaper and The Economist Group. All rights reserved.