April
1, 2009: The sunspot cycle is behaving a little like
the stock market. Just when you think it has hit bottom, it
goes even lower.
2008
was a bear. There were no sunspots observed on 266 of the
year's 366 days (73%). To find a year with more blank suns,
you have to go all the way back to 1913, which had 311 spotless
days: plot.
Prompted by these numbers, some observers suggested that the
solar cycle had hit bottom in 2008.
Maybe
not. Sunspot counts for 2009 have dropped even lower. As of
March 31st, there were no sunspots on 78 of the year's 90
days (87%).
It
adds up to one inescapable conclusion: "We're experiencing
a very Deep Solar Minimum, " says solar physicist Dean
Pesnell of the Goddard Space Flight Center.
"This
is the quietest sun we've seen in almost a century, "
agrees sunspot expert David Hathaway of the Marshall Space
Flight Center.
The sunspot cycle from 1995 to the present. The jagged curve
traces actual sunspot counts. Smooth curves are fits to the
data and one forecaster's predictions of future activity.
Credit: David Hathaway, NASA/MSFC. [more]
Quiet
suns come along every 11 years or so. It's a natural part
of the sunspot cycle, discovered by German astronomer Heinrich
Schwabe in the mid-1800s. Sunspots are planet-sized islands
of magnetism on the surface of the sun; they are sources of
solar flares, coronal mass ejections and intense UV radiation.
Plotting sunspot counts, Schwabe saw that peaks of solar activity
were always followed by valleys of relative calm—a clockwork
pattern that has held true for more than 200 years: plot.
The
current solar minimum is part of that pattern. In fact, it's
right on time. "We're due for a bit of quiet—and here
it is, " says Pesnell.
But
is it supposed to be this quiet? In 2008, the sun
set the following records:
A
50-year low in solar wind pressure: Measurements
by the Ulysses spacecraft reveal a 20% drop in solar wind
pressure since the mid-1990s—the lowest point since such measurements
began in the 1960s. The solar wind helps keep galactic cosmic
rays out of the inner solar system. With the solar wind flagging,
more cosmic rays are permitted to enter, resulting in increased
health hazards for astronauts. Weaker solar wind also means
fewer geomagnetic storms and auroras on Earth.
A
12-year low in solar "irradiance": Careful
measurements by several NASA spacecraft show that the sun's
brightness has dropped by 0.02% at visible wavelengths and
a whopping 6% at extreme UV wavelengths since the solar minimum
of 1996. These changes are not enough to reverse the course
of global warming, but there are some other, noticeable side-effects:
Earth's upper atmosphere is heated less by the sun and it
is therefore less "puffed up." Satellites in low
Earth orbit experience less atmospheric drag, extending their
operational lifetimes. That's the good news. Unfortunately,
space junk also remains longer in Earth orbit, increasing
hazards to spacecraft and satellites.
Space-age measurements of the total solar irradiance (brightness
summed across all wavelengths). This plot, which comes from
researcher C. Fröhlich, was shown by Dean Pesnell at the Fall
2008 AGU meeting during a lecture entitled "What is Solar
Minimum and Why Should We Care?"
A
55-year low in solar radio emissions: After World
War II, astronomers began keeping records of the sun's brightness
at radio wavelengths. Records of 10.7 cm flux extend back
all the way to the early 1950s. Radio telescopes are now recording
the dimmest "radio sun" since 1955: plot.
Some researchers believe that the lessening of radio emissions
is an indication of weakness in the sun's global magnetic
field. No one is certain, however, because the source of these
long-monitored radio emissions is not fully understood.
All
these lows have sparked a debate about whether the ongoing
minimum is "weird", "extreme" or just
an overdue "market correction" following a string
of unusually intense solar maxima.
"Since
the Space Age began in the 1950s, solar activity has been
generally high, " notes Hathaway. "Five of the ten
most intense solar cycles on record have occurred in the last
50 years. We're just not used to this kind of deep calm."
Deep
calm was fairly common a hundred years ago. The solar minima
of 1901 and 1913, for instance, were even longer than the
one we're experiencing now. To match those minima in terms
of depth and longevity, the current minimum will have to last
at least another year.
An artist's concept of NASA's Solar Dynamics Observatory.
Bristling with advanced sensors, "SDO" is slated
to launch later this year--perfect timing to study the ongoing
solar minimum. [more]
|
In
a way, the calm is exciting, says Pesnell. "For the first
time in history, we're getting to see what a Deep Solar Minimum
is really like." A fleet of spacecraft including the
Solar and Heliospheric Observatory (SOHO), the twin STEREO
probes, the five THEMIS probes, ACE, Wind, TRACE, AIM, TIMED,
Geotail and others are studying the sun and its effects on
Earth 24/7 using technology that didn't exist 100 years ago.
Their measurements of solar wind, cosmic rays, irradiance
and magnetic fields show that solar minimum is much more interesting
and profound than anyone expected.
Modern
technology cannot, however, predict what comes next. Competing
models by dozens of top solar physicists disagree, sometimes
sharply, on when this solar minimum will end and how big the
next solar maximum will be. Pesnell has surveyed the scientific
literature and prepared a "piano
plot" showing the range of predictions. The great
uncertainty stems from one simple fact: No one fully understands
the underlying physics of the sunspot cycle.
Pesnell
believes sunspot counts will pick up again soon, "possibly
by the end of the year, " to be followed by a solar maximum
of below-average intensity in 2012 or 2013.
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