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.

Their report concerns variations in the speed of parcels of magnetized hot gas that travel from the Sun’s equatorial regions towards the poles. These “meridional flows” carry north-polar magnetism to the south magnetic pole, and vice versa, thus swapping the magnetic poles around from one sunspot cycle to the next. A difficulty for Hathaway and Rightmire was that the magnetic streams flow very slowly by solar standards, at usually less than 50 km per hour, while other features move 200 times faster. Yet by comparing 60,000 pairs of magnetic images, each separated in time by 8 hours, these analysts have been able to detect changes in speed in the poleward magnetic streams during Cycle 23, and a prolonged fast flow in recent years.

Variation of speeds of poleward flow on the Sun,1996-2009, as measured by SOHO/MDI. The sunspot number in red (smoothed and divided by 20) shows Solar Cycle 23 rising to its maximum 2000-02, and then declining. Note the relatively fast flow during the approach to the present prolonged minimum. From Hathaway & Rightmire, Science 2010, distributed by ESA and NASA.

At first sight it’s counter-intuitive that relatively fast magnetic streams should delay the switch in the Sun’s magnetic poles that’s needed to trigger the next solar cycle. Hathaway and Rightmire account for the outcome by saying that the fast flow swept along with it parcels of the wrong magnetic polarity. “This requires a longer time to reverse the old polar fields and builds up weaker polar fields of the opposite polarity.”

The explanation will be of some comfort to David Hathaway, whose attempts to forecast the onset of Cycle 24 were repeatedly frustrated by the Sun’s reluctance to conform to them. And that brings us back to Jeff Kuhn’s wish for better understanding of the Sun for the sake of predicting the climate on Earth.

What did Henrik Svensmark and I say about this challenge in The Chilling Stars? The fact that Hathaway and Rightmire’s measurements extend only from 75 degrees south latitude to 75 north, excluding the Sun’s polar regions where the physical decisions about the next cycle are made, typifies in our opinion the present problem for solar physicists. So in the chapter entitled “The Agenda for Cosmoclimatology” Svensmark and I were obliged to look forward to new concepts in solar space missions.

Would-be predictors of the Sun’s future behaviour are frustrated by the difficulty of gauging the present strength of its main magnetic field. That is because the regions around the poles are seen almost edge-on from the Earth and from most spacecraft. The Ulysses spacecraft orbits over the poles and measures the magnetic field surrounding it in space, but it does not carry the right instruments to measure the field at the surface by remote sensing. This lack will be put right by future space missions.

Europe’s Solar Orbiter will use encounters with Venus over seven years to manoeuvre itself into a position from which it can see one of the poles from a slant angle of 38 degrees, compared with 7 degrees from the Earth. There is also a proposal for a Solar Polar Orbiter to be powered by solar sails and eventually to circle over the north and south poles of the Sun at half the Earth–Sun distance. That would give solar physicists a comprehensive view of solar magnetism at the visible surface for the very first time, and the hope of better prediction of the Sun’s behaviour is advertised as the primary motive for such a mission.

Don’t hold your breath. The Solar Orbiter is not due for launch until about 2015 and it will not have a good view of the pole until 2020. As for the Solar Polar Orbiter, it’s little more than a gleam in the eye of its advocates. Although the European Space Agency has adopted it as one of the novel things to try to do in the period 2015–25, the scientists and engineers involved may count themselves lucky if their solar sailor is in orbit around the Sun by end of that period.

Updating the remarks about the various space projects:

• After 16 years in its solar-polar orbit Ulysses completed its mission in 2008.
• So far from being launched “about 2015”, Solar Orbiter is now in competition with Euclid (dark energy) and PLATO (habitable planets) as a candidate to be one of two ESA medium-class missions to be launched no earlier than 2017.
• As for Solar Polar Orbiter, a study of the proposed mission was completed in 2004, but otherwise I can find no mention of it in ESA’s science website since 2005. NASA had a similar study project called Solar Polar Imager or Polaris, but now it is missing entirely from NASA’s science website.

ESA's concept for a Solar Polar Orbiter uses a solar sail to manoeuvre to a commanding view of the Sun's poles. Notice the optimistic time-line, 2010-15, in this 2004 study. Planetary and Solar System Exploration Studies Section, ESA.

If the major role of the Sun’s magnetic activity in climate change is widely recognized again before too long, the apparently off-hand attitude of the space agencies to these solar polar projects will look strange to future historians of science. As ESA’s own Science Advisory Committee declared unequivocally in its report Cosmic Vision (2005)

The structure of the global magnetic field at the Sun’s surface is not known and its determination will require observations from above the poles. … The primary requirement is for a solar polar orbiter.

Meanwhile we’ll all just have to guess what the Sun will do next.

References

M. Emilio, J.R. Kuhn and R.I. Bush, “One solar cycle of solar astrometry with MDI/SOHO” in “Solar and Stellar Variability: Impact on Earth and Planets,” Proceedings of the International Astronomical Union, IAU Symposium, Vol. 264, pp. 21-32, Cambridge UP

U. Hawaii press release http://www.ifa.hawaii.edu/info/press-releases/SunSize-May2010/

D. H. Hathaway and L. Rightmire, “Variations in the Sun’s Meridional Flow over a Solar Cycle”, Science, Vol. 327, pp. 1350-52, 2010

H. Svensmark and N. Calder, The Chilling Stars, pp. 223-4, Icon Books 2007/2008

Cosmic Vision: Space Science for Europe 2015-2025, European Space Agency, BR-247, 2005

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