Just
looking at the number of sunspots doesn't provide a full picture of how the
sun's solar energy impacts Earth, a new study suggests. The findings contradict
previous thinking about how the sun behaves during low points in its solar
cycle.
"What
we're realizing is that the sunspots
do not tell the whole story," said Sarah Gibson, a scientist from the National Center for Atmospheric Research (NCAR) in Boulder, Colo.
Sunspots
are areas of concentrated magnetic activity that appear as dark dots on the
sun's surface. The number of spots periodically rises and falls in what has
become known as the solar cycle. This cycle lasts about 11 years, taking
roughly 5.5 years to go from "solar minimum," or a period of time
when there are few sunspots, to the cycle peak, or "solar maximum,"
during which there are many sunspots.
During
cycle peaks — the next one is expected
in 2013 — there are frequent solar
flares and geomagnetic storms, events that send out radiation that can
bombard the Earth's atmosphere, damaging satellites and disrupting power grids.
Scientists say that a really bad solar storm, akin to one that started
fires along telegraph lines in 1859, could bring
modern society to its knees.
Conversely,
cycle minimums were thought to be very quiet times, periods when the Earth
would not experience as many blasts of solar energy. But Gibson's study shows
that this is not necessarily the case.
Full
force in 2008
Gibson and
her colleagues compared measurements from two different cycle minimums — one
from 1996 and one from 2008. They analyzed a type of solar energy called the
solar wind — streams of charged particles that accelerate out from the sun's
extremely hot atmosphere. The solar wind, unlike short-lived solar storms,
streams from the sun pretty much constantly but with varying intensity.
They found
that, while the solar winds intersecting the Earth largely disappeared in 1996,
they continued to hit the Earth at full force in 2008.
These
results show that "what we thought was a typical solar minimum wasn't, and
what we're seeing now is a different animal," Gibson said.
Scientists
previously thought that during solar minimums, solar winds would simply blow
out the top and the bottom of the sun, and not come out near the equator. Since
the Earth is close to the same latitude as the sun's equator, it shouldn't
experience much solar wind during a low point in the solar cycle.
"If
you imagine holding a hose and you hold it straight up, you would spray up, and
if you had a friend standing nearby, they might get a little wet, but not
soaked," Gibson said. But during the current solar minimum in 2008, the
fire hose was still pointing at the Earth.
"The
last two solar minimum, this didn't happen. When the sun spots went away, these
fire hoses went away too," she said.
In fact,
the solar wind's effect on the Earth's radiation belt — a ring of charged
particles around the planet — was three times greater in 2008 than in 1996.
While the effects of solar winds aren't as drastic as those of a solar flare,
they can still interfere with satellite orbits and radio communications.
Changing
as you read this
This year,
the solar winds are starting to taper off. However, Gibson was surprised that
the reduction of winds lagged so far behind the decrease in sunspots.
What could
account for this difference in minimums? Gibson thinks it may have something to
do with the fact that this cycle's minimum has been historically wimpy — there
were fewer
sunspots during this minimum than during any minimum in the last 75 years.
"In a
minimum when you have a really strong polar field, it can clamp down everything
else at lower latitudes down towards the equator, and really the only action is
what's coming out the poles, and you get what we thought was a classic solar
minimum picture, and you don't get any wind escaping," Gibson said. "But
because we have such a weak magnetic field this cycle compared to last cycle
minimum ... the polar field is not as strong, it can't clamp down, and stuff kind
of escapes, you get these streams that squirt out at lower latitudes near the
equator instead of just at the poles," she said.
Scientists
are trying to learn more about the solar cycle to understand why it occurs and
what accounts for cycle differences.
The solar
cycle is "something that's become very important to understand in the
space age because we have all these satellites, we have astronauts out there,
and you have to know what space whether they're likely to face," Gibson
said.
The study
was led by Gibson, and the research team included scientists from NCAR's High
Altitude Observatory, the University of Michigan, NOAA and NASA. The results
will appear this week in the Journal of Geophysical Research. The
research was funded by NASA and the National Science Foundation.
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