Sun still sulks

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The Sun still sulks

Two magnetograms from the ESA-NASA SOHO spacecraft contrast the Sun’s liveliness of exactly 10 years ago (20 June 2000) on the left with its feeble performance today (20 June 2010) on the right. In these images made with Stanford’s Michelson Doppler Interferometer, north magnetic polarity is white, south magnetic polarity is black.

Solstice sunrise over Stonehenge 2005. Credit: User: Solipsist.

As many thousands flock to Stonehenge for tomorrow’s summer solstice, this is a moment to ask for the umpteenth time what the Sun is up to. The mean sunspot number in June 2000 was 119, today it is 28, with the spots clustered in the northern region showing most magnetic activity. Since 2004 there have been 803 days with no sunspots at all (35 in 2010, 260 in 2009). During a typical sunspot minimum there are fewer than 500 spotless days.

In the current issue of the Royal Astronomical Society’s magazine Astronomy and Geophysics, Nigel Weiss of Cambridge considers the long-term variability of the Sun and alternative theories about it, especially concerning “grand maxima” in activity like that in the 20th Century, and “grand minima” like the Maunder Minimum of 300 years ago associated with the Little Ice Age. Weiss’s conclusion is that there’s a 40 % chance the current grand maximum will be followed by a grand minimum.

As for the climatic implications, Weiss and I agreed to differ some years ago. Although we both say that the Intergovernmental Panel on Climate Change underestimates the influence of the Sun, Weiss thinks it can’t compete with man-made global warming. His article ends:

Even if the Sun does enter a new Maunder-like grand minimum, any cooling effect will be small compared with the warming produced by anthropogenic greenhouse gases.

Contrast that with Henrik Svensmark’s conclusion in an article for the Danish newspaper Jyllands-Posten.

That the Sun might now fall asleep in a deep minimum was suggested by solar scientists at a meeting in Kiruna in Sweden two years ago. So when Nigel Calder and I updated our book The Chilling Stars, we wrote a little provocatively that “we are advising our friends to enjoy global warming while it lasts.”

In fact global warming has stopped and a cooling is beginning. Mojib Latif from the University of Kiel argued at the recent UN World Climate Conference in Geneva that the cooling may continue through the next 10 to 20 years. His explanation was a natural change in the North Atlantic circulation, not in solar activity. But no matter how you interpret them, natural variations in climate are making a comeback.

The outcome may be that the Sun itself will demonstrate its importance for climate and so challenge the theories of global warming. No climate model has predicted a cooling of the Earth – quite the contrary. And this means that the projections of future climate are unreliable. A forecast saying it may be either warmer or colder for 50 years is not very useful, and science is not yet able to predict solar activity.

So in many ways we stand at a crossroads. The near future will be extremely interesting. I think it is important to accept that Nature pays no heed to what we humans think about it. Will the greenhouse theory survive a significant cooling of the Earth? Not in its current dominant form. Unfortunately, tomorrow’s climate challenges will be quite different from the greenhouse theory’s predictions. Perhaps it will become fashionable again to investigate the Sun’s impact on our climate.

References

N. Weiss, “Modulation of the Sunspot Cycle”, Astronomy and Geophysics, Vol. 51, pp. 3.9-3.15, 2010

H. Svensmark: “While the Sun sleeps” (in Danish), Jyllands-Posten, 9 September, 2009

For a related post on this blog see https://calderup.wordpress.com/2010/05/12/puzzling-sun/

Postscript on the Song of the Sun

I see that Sheffield solar physicists now generate music from observations of the magnetic coronal loops. Read about it (and hear it): http://www.telegraph.co.uk/science/space/7840201/Music-of-the-sun-recorded-by-scientists.html

For an earlier Song of the Sun, using its internal vibrations seen by SOHO’s MDI, click on the second item here (but beware – it’s about 18 MB with visuals) http://www.esa.int/esaSC/SEMLAJWO4HD_index_0.html

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Puzzling Sun

Climate Change – Updating The Chilling Stars

The deeply puzzling Sun

“We can’t predict the climate on Earth until we understand these changes on the Sun.” So says Jeff Kuhn, who runs the Haleakala Observatories of Hawaii’s Institute for Astronomy, and his sentiment will be shared by anyone who thinks that the Sun plays a major part in climate change.

Impression of the long-lived SOHO. ESA & NASA

He makes the comment in a press release (11 May 2010) about a remarkably small change in the Sun’s diameter in the course of the most recent sunspot cycle, Cycle 23. With colleagues from Universidade Estadual de Ponta Grossa (Brazil) and Stanford University, Kuhn reports in an International Astronomical Union paper:

“… the method and results of precise solar astrometry made with the Michelson Doppler Imager (MDI), on board the Solar and Heliospheric Observatory (SOHO), during one complete solar cycle. We measured an upper limit to the solar radius variation, the absolute solar radius value and the solar shape. Our results are 22 [milli-arcseconds] peak-to-peak upper limit for the solar radius variation over the solar cycle, the absolute radius was measured as 959.28 ± 0.15 [arcseconds] at 1 [astronomical unit], and the difference between polar and equatorial solar radii in 1997 was 5 km and about three times larger in 2001.”

In plain language, the visible Sun’s diameter changed by less than one millionth during 12 years of observation. That’s despite the daily frenzy of solar activity and the great contrasts in behaviour during the maxima and minima of the sunspot counts. Kuhn hopes for even more precise measurements with NASA’s newly launched Solar Dynamics Observatory, but to see long-term changes you must obviously watch for a long time. It’s the durability of the ESA/NASA SOHO spacecraft and its MDI instrument, since the launch in December 1995, that makes the present results possible.

MDI is widely known for its daily images showing us where the sunspots are. At the time of this posting on12 May the Sun’s face is spotless, and the extraordinary wait continues for our lazy star to get going in earnest with its new Cycle 24.

MDI also peers into the solar interior by “helioseismology” and can even detect the presence of sunspots on the Sun’s far side. What’s more, MDI measures the line-of-sight magnetic field at the visible surface, thanks to which David Hathaway of NASA Huntsville and Lisa Rightmire of the University of Memphis can describe another change during Cycle 23.