Kinks in the Sun’s magnetic field have puzzled scientists
since they first started studying the solar wind, and now
researchers at NASA’s Jet Propulsion Laboratory, Pasadena,
Calif., have found the reason: they are caused by the
evolution of a type of magnetic wave called Alfven waves.

Scientists measured sudden changes in the Sun’s magnetic
field with the magnetometer instrument on the Ulysses
spacecraft, which is orbiting the Sun’s poles at a distance
between Jupiter and Mars. Ulysses has been studying the Sun
since 1990 and has just finished studying the south pole of
the Sun at solar maximum, a time of great activity.

“Over the poles of the Sun, we saw abrupt decreases in
the magnetic field,” said JPL’s Dr. Bruce Tsurutani, a co-
investigator on the magnetometer instrument on Ulysses. “We
did not know what they were, because we had never seen
anything like it before. Now we know that the disturbance is
caused by Alfven waves.”

Scientists expected to find that either the field
magnitude remained the same, though the angle changed, or that
the magnitude changed, with no fields threading across the
structure, said Tsurutani. Instead, they found that the ends
of Alfven waves always have both rotational and tangential

Like the movements of a plucked guitar string, Alfven
waves travel down the magnetic fields that emanate from the
Sun. Disturbances in the Sun’s magnetic field, which is
embedded in the solar wind, travel through space to eventually
cause auroras on Earth. The high-energy particles from the
solar wind become trapped in the Earth’s magnetic field and
come down into the atmosphere near the Earth’s north and south
magnetic poles. The highly-charged particles then collide
with oxygen and nitrogen in Earth’s atmosphere and emit light,
forming the aurora.

Tsurutani also studied polar plumes, long trails from the
base of the Sun. The plumes form in the Sun’s polar regions,
the upper and lower 30-degree latitude regions, and where
these plumes occur, the magnetic field isn’t kinked, but
instead forms long, thin, straight tubes. This means that the
Alfven waves don’t operate in these regions, though scientists
don’t yet know why.

“Ulysses was able to find that the Sun’s polar plumes
stretch out past the orbit of Mars and maybe farther,” said
Tsurutani. “What’s fascinating is how these plumes can be so
thin and so long at the same time.” A plume could be 100 times
wider than it is long. The European Space Agency’s Solar and
Heliospheric Observatory (SOHO) noted these polar plumes in

Alfven waves are named for Hannes Alfven, a Swede who
in1942 discovered the waves, for which he was later awarded
the Nobel Prize.

Tsurutani discussed his findings this week at the
European Geophysical Society’s 26th annual meeting, joined by
his colleagues on the study, Dr. Carlos Galvan, Dr. John
Arballo, Dr. Regina Sakurai and Dr. Daniel Winterhalter, from
the Space Plasma Physics Element at JPL, Dr. Bimla Buti
University of New Delhi and Dr. Gurbax Lakhina, director of
the Mumbai Geomagnetic Observatory, Bombay, India.

Ulysses, launched in 1990, is a joint venture of NASA and
the European Space Agency. JPL manages Ulysses for NASA’s
Office of Space Science, Washington, D.C. More information on
the Ulysses mission is available at the JPL Ulysses website: and ESA’s Ulysses website, . JPL is managed by the
California Institute of Technology in Pasadena for NASA.