Something strange happened on the sun last week: all the
sunspots vanished. This is a sign, say scientists, that solar minimum
is coming sooner than expected.
by Dr Tony Phillips
Six â€¦ long â€¦ years.
Solar physicist David Hathaway has
been checking the sun every day since 1998, and every day for six
years there have been sunspots. Sunspots are planet-sized "islands"
on the surface of the sun. They are dark, cool, powerfully magnetized,
and fleeting: a typical sunspot lasts only a few days or weeks before
it breaks up. As soon as one disappears, however, another emerges
to take its place.
Even during the lowest ebb of solar
activity, you can usually find one or two spots on the sun. But
when Hathaway looked on Jan. 28, 2004, there were none. The sun
was utterly blank.
It happened again last week, twice,
on Oct. 11th and 12th. There were no sunspots.
"This is a sign," says Hathaway,
"that the solar minimum is coming, and it's coming sooner than we
Solar minimum and solar maximum - "Solar
Min" and "Solar Max" for short - are two extremes of the sun's 11-year
activity cycle. At maximum, the sun is peppered with spots, solar
flares erupt, and the sun hurls billion-ton clouds of electrified
gas toward Earth. It's a good time for sky watchers who enjoy auroras,
but not so good for astronauts who have to be wary of radiation
storms. Power outages, zapped satellites, malfunctioning GPS receivers - these
are just a few of the things that can happen during Solar Max.
The blank sun on Oct. 11, 2004, photographed by the ESA/NASA
Solar and Heliospheric Observatory.
Solar minimum is different. Sunspots
are fewer - sometimes days or weeks go by without a spot. Solar flares
subside. It's a safer time to travel through space, and a less interesting
time to watch polar skies.
Hathaway is an expert forecaster
of the solar cycle. He keeps track of sunspot numbers (the best
known indicator of solar activity) and predicts years in advance
when the next peaks and valleys will come. It's not easy:
"Contrary to popular belief," says
Hathaway, "the solar cycle is not precisely 11 years long." Its
length, measured from minimum to minimum, varies: "The shortest
cycles are 9 years, and the longest ones are about 14 years." What
makes a cycle long or short? Researchers aren't sure. "We won't
even know if the current cycle is long or short - until it's over,"
But researchers are making
progress. Hathaway and colleague Bob Wilson, both working at NASA's
Marshall Space Flight Centre, believe they've found a simple way
to predict the date of the next solar minimum. "We examined data
from the last 8 solar cycles and discovered that Solar Min follows
the first spotless day after Solar Max by 34 months," explains Hathaway.
The most recent solar maximum was
in late 2000. The first spotless day after that was Jan 28, 2004.
So, using Hathaway and Wilson's simple rule, solar minimum should
arrive in late 2006. That's about a year earlier than previously
Credit Pat Rawlings
Robot moonship: an artist's concept.
The next solar maximum might come
early, too, says Hathaway. "Solar activity intensifies rapidly after
solar minimum. In recent cycles, Solar Max has followed Solar Min
by just 4 years." Do the math: 2006 + 4 years = 2010.
By that time, according to NASA's
for space exploration, robot ships will be heading for the moon
in advance of human explorers. If Hathaway and Wilson's prediction
is correct, those robots will need good shields. Solar flares and
radiation storms can damage silicon brains and electronic guts almost
as badly as their organic counterparts.
For now, says Hathaway, we're about
to experience "the calm before the storm." And although he's a fan
of solar activity - what solar physicist isn't? - he's looking forward
to the lull. "It'll give us a chance to see if our 'spotless sun'
method for predicting solar minimum really works."
Solar Max will be back soon enough.