Puzzling Sun

12/05/2010

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.

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