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July
11, 2008: Stop the presses! The sun is behaving normally.
So
says NASA solar physicist David Hathaway. "There have
been some reports lately that Solar Minimum is lasting longer
than it should. That's not true. The ongoing lull in sunspot
number is well within historic norms for the solar cycle."
This
report, that there's nothing to report, is newsworthy because
of a growing buzz in lay and academic circles that something
is wrong with the sun. Sun Goes Longer Than Normal Without
Producing Sunspots declared one recent press release.
A careful look at the data, however, suggests otherwise.
But
first, a status report: "The sun is now near the low
point of its 11-year activity cycle," says Hathaway.
"We call this 'Solar Minimum.' It is the period of quiet
that separates one Solar Max from another."
Above:
The solar cycle, 1995-2015. The "noisy" curve traces
measured sunspot numbers; the smoothed curves are predictions.
Credit: D. Hathaway/NASA/MSFC. [more]
During
Solar Max, huge sunspots and intense solar flares are a daily
occurrence. Auroras appear in Florida. Radiation storms knock
out satellites. Radio blackouts frustrate hams. The last such
episode took place in the years around 2000-2001.
During
Solar Minimum, the opposite occurs. Solar flares are almost
nonexistent while whole weeks go by without a single, tiny
sunspot to break the monotony of the blank sun. This is what
we are experiencing now.
Although
minima are a normal aspect of the solar cycle, some observers
are questioning the length of the ongoing minimum,
now slogging through its 3rd year.
"It
does seem like it's taking a long time," allows Hathaway,
"but I think we're just forgetting how long a solar minimum
can last." In the early 20th century there were periods
of quiet lasting almost twice as long as the current spell.
(See the end notes for an example.)
Most researchers weren't even born then.
Hathaway
has studied international sunspot counts stretching all the
way back to 1749 and he offers these statistics: "The
average period of a solar cycle is 131 months with a standard
deviation of 14 months. Decaying solar cycle 23 (the one we
are experiencing now) has so far lasted 142 months--well within
the first standard deviation and thus not at all abnormal.
The last available 13-month smoothed sunspot number was 5.70.
This is bigger than 12 of the last 23 solar minimum values."
In
summary, "the current minimum is not abnormally low or
long."
The
longest minimum on record, the Maunder Minimum of 1645-1715,
lasted an incredible 70 years. Sunspots were rarely observed
and the solar cycle seemed to have broken down completely.
The period of quiet coincided with the Little Ice Age, a series
of extraordinarily bitter winters in Earth's northern hemisphere.
Many researchers are convinced that low solar activity, acting
in concert with increased volcanism and possible changes in
ocean current patterns, played a role in that 17th century
cooling.
For
reasons no one understands, the sunspot cycle revived itself
in the early 18th century and has carried on since with the
familiar 11-year period. Because solar physicists do not understand
what triggered the Maunder Minimum or exactly how it influenced
Earth's climate, they are always on the look-out for signs
that it might be happening again.
The
quiet of 2008 is not the second coming of the Maunder Minimum,
believes Hathaway. "We have already observed a few sunspots
from the next solar cycle," he says. (See Solar
Cycle 24 Begins.) "This suggests the solar cycle
is progressing normally."
What's
next? Hathaway anticipates more spotless days1, maybe even
hundreds, followed by a return to Solar Max conditions in
the years around 2012.
Stay
tuned to Science@NASA for updates.
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Author: Dr.
Tony Phillips | Credit: Science@NASA
more
information: Spotless Days |
1Another
way to examine the length and depth of a solar minimum
is by counting spotless days. A "spotless day"
is a day with no sunspots. Spotless days never happen
during Solar Max but they are the "meat and potatoes"
of solar minima.
Adding
up every daily blank sun for the past three years, we
find that the current solar minimum has had 362 spotless
days (as of June 30, 2008).
Compare that value to the total spotless days of the
previous ten solar minima: 309, 273, 272, 227, 446,
269, 568, 534, ~1019 and ~931. The current count of
362 spotless days is not even close to the longest.
The
plot below compares the Solar Minimum of 2008 to a longer
one in 1933:
Orange
bars represent the number of spotless days per month.
The ongoing solar minimum needs to accumulate another
206 spotless days before it matches the duration of
the 1933 minimum, which is considered unremarkable by
solar historians.
What
does a spotless day look like? The Solar and Heliospheric
Observatory (SOHO) recorded this blank sun on July 1,
2008:
Click
here for a Solar Max comparison.
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