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Why
does the noise level change & what causes the noise ? |
Scientists track solar cycles
by counting sunspots -- cool planet-sized areas on the Sun where intense
magnetic loops poke through the star's visible surface. Hathaway is
an expert forecaster of sunspot numbers. "Sunspot counts peaked
in 2000 some months earlier than we expected," he recalls. The
subsequent dip toward solar minimum seemed premature to Hathaway,
and indeed it was. Before long, sunspot counts reversed course and
began to climb toward a second maximum that now appears to be only
a few percent smaller than the first.
Solar Max eleven years ago was much the same. A first peak arrived
in mid-1989 followed by a smaller maximum in early 1991. In fact,
if the ongoing cycle proves to be a double, it will be the third such
double-peaked cycle in a row.
T he last two sunspot cycles also had double-featured maxima.
During solar maximum, magnetic fields above the Sun's surface become
impressively tangled, particularly near sunspots. Twisted magnetic
fields -- stretched like taut rubber bands -- can snap back and explode,
powering solar flares and coronal mass ejections.
Sunspots are the most visible sign of those complex magnetic fields
-- but not the only one. Another sign is solar radio emissions, which
come from hot gas trapped in magnetic loops. "The radio Sun is
even brighter now than it was in 2000," says Hathaway. By the
radio standard, this second peak is larger than the first.
Hathaway notes a widespread misconception that solar activity varies
every 11 years "like a pure sinusoid." In fact, he says,
solar activity is chaotic; there is more than one period.
Earth-directed solar explosions, for instance, tend to happen every
27 days -- the time it takes for sunspots to rotate once around the
Sun. There is also an occasional 155-day cycle of solar flares. No
one knows what causes it. And the double peaks of recent solar maxima
are separated by approximately 18 months. |
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