Ulysses results inspire a big discovery about the Sun's behaviour
03 June 1999The strength of the Sun's magnetic field has doubled during the 20th Century, according to calculations by British scientists. This finding will help to clarify the Sun's contribution to climate change on the Earth. A team at the Rutherford Appleton Laboratory near Oxford has been able to work out the recent history of the Sun's magnetic behaviour, thanks to the unprecedented overview of solar magnetism provided by the ESA-NASA spacecraft Ulysses.
"One surprise led to another," says Mike Lockwood, lead author of the new report. "Ulysses found that the radial component of the magnetic field far out from the Sun is equally strong at all solar latitudes. Nobody expected that, but it means we can use historical data from just one place, the Earth, to deduce a surprising change for the whole Sun. The Ulysses result was absolutely crucial."
Launched in 1990 and still going strong, Ulysses is the first spacecraft ever to pass over the polar regions of the Sun. It revealed that the solar wind of electric particles is generally much faster than that coming from the Sun's equatorial regions, which supply the solar wind felt in the Earth's vicinity. But the strength of the magnetic field carried by the solar wind remains stubbornly constant. Over the Sun's south pole in 1994, and in a quick transit to the north pole in 1995, Ulysses showed that the solar wind smooths out an expected intensification in the polar regions.
Encouraged by this result, Lockwood and his team re-examined a record of magnetic storms on the Earth provoked by the Sun. Called the "aa" index, it comes from simultaneous observations of magnetic events in England and Australia. The Rutherford group found that recent values of the index match very closely the variations in the strength of the solar magnetic field as measured by spacecraft. Since 1964, the magnetic field has intensified by 40 per cent.
As the longest of all records in solar-terrestrial physics, the "aa" index goes back to 1868. Repeated increases and falls correspond roughly with the cycles of sunspots counted on the Sun's visible surface. More remarkable is a rising trend in the index through most of the 20th Century. The Rutherford team deduces from the trend an overall increase in the Sun's magnetic field by a factor of 2.3, since 1901.
Eugene Parker of the University of Chicago, the father of solar-wind theory, comments on the result: "It is a historical fact that our capricious climate responds to variations of the Sun's magnetic activity, with substantial warming and cooling with the rise and fall of activity over the centuries." In Parker's opinion the new discovery about solar magnetism should prompt fresh attention to the role of the Sun in contemporary climate change, alongside any effect due to man-made carbon dioxide.
The Rutherford team itself is already using the magnetic data to deduce increases in the Sun's brightness during the 20th Century. Other work in Europe on solar influences on climate includes studies of stratospheric effects (Berlin and Leicester) and of changes in cloud cover apparently associated with variations in cosmic rays, which obey changes in the solar wind (Danish Space Research Institute).
The paper, "A doubling of the Sun's coronal magnetic field during the past 100 years" by M. Lockwood, R. Stamper and M.N. Wild, is published in the journal Nature, 3 June 1999, vol. 399, pp. 437-9. The comments by E.N. Parker are in the same issue, pp. 416-7.