Solar activity heads for lowest low in four centuries

The sun's activity is in free fall, according to a leading space physicist. But don't expect a little ice age. "Solar activity is declining very fast at the moment," Mike Lockwood, professor of space environmental physics at Reading University, UK, told New Scientist. "We estimate faster than at any time in the last 9300 years."

Lockwood and his colleagues are reassessing the chances of this decline continuing over decades to become the first "grand solar minimum" for four centuries. During a grand minimum the normal 11-year solar cycle is suppressed and the sun has virtually no sunspots for several decades. This summer should have seen a peak in the number of sunspots, but it didn't happen.

Lockwood thinks there is now a 25 per cent chance of a repetition of the last grand minimum, the late 17th century Maunder Minimum, when there were no sunspots for 70 years. Two years ago, Lockwood put the chances of this happening at less than 10 per cent (Journal of Geophysical Research, DOI: 10.1029/2011JD017013).

                                             Snow fell in Egypt for the first time after 100 years

   Little ice age

The Maunder Minimum coincided with the worst European winters of the little ice age, a period lasting centuries when several regions around the globe experienced unusual cooling. Tree ring studies suggest it cooled the northern hemisphere by up to 0.4 °C.

But Lockwood says we should not expect a new grand minimum to bring on a new little ice age. Human-induced global warming, he says, is already a more important force in global temperatures than even major solar cycles. Temperatures have risen by 0.85 °C since 1880, with more expected, according to the most recent assessment of the Intergovernmental Panel on Climate Change.

There may still be noticeable consequences. For instance, long term cold winters in the UK are common when solar activity is low. And less solar activity can slow the jet stream, triggering a suite of interlinked extreme weather events like the Russian heatwave of 2010, and the devastating floods in Pakistan that same year.

   Isotope trail

There have been 24 grand solar minima in the past 10,000 years. Their history is reconstructed by looking for isotopes like carbon-14 that cosmic rays generate in the atmosphere. Solar activity boosts the solar wind which deflects cosmic rays coming at Earth, so less solar activity means more cosmic rays and more of these isotopes.

How the isotopes vary over time can be measured by looking at things like tree rings, which absorb carbon-14, or ice cores, which accumulate beryllium-10.

                                    Recently published planetary EM index that tracks the solar cycle

The current long-term decline in solar activity set in after the last grand solar maximum peaked in 1956, says Lockwood. The decline has accelerated recently, and the absence of sunspots this summer has set alarm bells ringing.

The precise extent to which solar cycles influence global temperatures is still debated, including whether the recent decline may have helped cause the current hiatus in the pace of global warming.

"Mike is probably right that there is a chance of the sun returning to a level of activity similar to the Maunder Minimum," says atmospheric physicist Joanna Haigh of Imperial College. But she adds: "Even under the most optimistic scenario [of minimal global warming and a deep solar minimum] the solar cooling would only just offset greenhouse gas warming. So no ice age."

It is more likely that it will simply reduce the warming a little, and set us up for greater warming if it receded.

   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.

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.

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 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.

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.

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