Deep Solar Minimum
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.
Above: 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.
|
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 6% at extreme UV wavelengths since the solar minimum of 1996.
The changes so far are not enough to reverse the course of global warming, but
there are some other significant 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. Unfortunately, space junk also remains longer in Earth orbit,
increasing hazards to spacecraft and satellites.
Above: 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.
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, Hinode, 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.
But
like other forecasters, he knows he could be wrong. Bull or bear? Stay tuned for
updates.
This is what makes summers cooler and winters more extreme so yeah this is interesting.
