ESA Science News

26 Nov 1999

G-modes detected in interplanetary magnetic field?

New evidence that gravity waves originating in the Sun’s core may leave their imprint in the solar wind was presented to
last month’s meeting of the Ulysses science working team.

David Thomson and Louis Lanzerotti from Bell Labs, Lucent Technologies, New Jersey, USA told the meeting in Shelter
Island, San Diego that they have found the tell tale signs of g modes in magnetic field data collected by the magnetometer
experiment on board ESA’s Ulysses spaceprobe.

The detection of g modes is one of the major prizes in solar physics. But despite years of searching, nobody has ever seen
them on the Sun’s surface. In 1995, however, Thomson, Lanzerotti, and their colleague Carol Maclennan published a
paper in Nature claiming they had found periodicities in low energy particle data collected by Ulysses that matched
predicted g mode frequencies. Now they are saying they have found similar frequencies in an analysis of the
interplanetary magnetic field.

This latest analysis adds weight to the earlier work. But it is also re-kindling the controversy that greeted the Nature
paper. Reservations focused mainly on the interpretation of the periodicities rather than their existence, with some
critics saying they could as easily be understood as random fluctuations as due to g modes. “It would be extremely
exciting if they had found g modes because it would tell us about the interior of the Sun,” says Douglas Gough, Professor
of Theoretical Astrophysics at Cambridge University, UK. “But I’m not convinced that the interpretation is correct.”

For the earlier work, Thomson, a statistician, analysed Ulysses HI-SCALE particle flux data. Now he has carried out a
complementary statistical analysis on magnetic field data and has come up with the same result. A paper published in
Astrophysical Journal earlier this year prompted the latest Bell Labs work. The paper, which reported a different
statistical analysis of the magnetic field data by a team at Imperial College, London, came to a different conclusion —
that the frequencies found were consistent with statistical fluctuations and not g modes. The Bell Labs group told the
meeting that the contrary results occurred because the Imperial College group used a wider bandwidth (a wider spread of
particle energies and magnetic field strengths) than did Thomson in both the original work and the new results.

Thomson’s analyses have found that about half the energy in the solar wind is in discrete modes. This is contrary to the
widely held belief that the solar wind must be chaotic. “Once we looked at Ulysses data, which comes in long
uninterrupted stretches, we started seeing things in there that are strongly periodic. What can be periodic on the
timescale we observed of a few hours to days? G modes,” he says.

Lanzerotti, a solar physicist and Thomson’s collaborator, was surprised when the frequencies first emerged. “I was
surprised, shocked — I couldn’t believe it,” he says. “My mind was fixed — the interplanetary medium is random,
chaotic. But now I’m convinced it isn’t chaotic and has a deterministic component of about 50%. The source for that has to
be the Sun.” The debate hinges not only on the statistical interpretation, but also on the physical mechanism that could be
responsible for imprinting g modes on the solar wind. “Gravity waves spend a lot of time in the Sun’s centre and
penetrate very little into the convection zone,” says Gough, which explains why they are so difficult to observe on the
surface. However, he can envisage mechanisms by which they could influence the interplanetary medium. “If the g modes
were pushing the magnetic field back and forth on the surface, we might see them in the solar wind. Only further, more
detailed analysis that looks for expected patterns in g modes across the Sun would settle the controversy once and for
all,” he says.

Another intriguing finding was also presented to the Science Working Team meeting about the possible identification of a
comet tail. A report will follow shortly to coincide with publication in the scholarly literature.


* More about Ulysses

* Ulysses: the Sun around us