FWD: [UASR] Ulysses Results Inspire A Big Discovery About The Sun's Behaviour
Posted by : "Mark A. LeCuyer" <randydan@...>
Source: ESA Science News
June 3, 1999
Ulysses Results Inspire A Big Discovery About The Sun's Behaviour
The 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
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
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.
Useful links for this story:
* Ulysses - the Sun around us
* Ulysses Science home page
Images supporting this release are available at
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