|An urgent need
to understand the Sun's magnetic behaviour, which produces space storms
affecting the Earth, gives practical as well as theoretical importance
to the discovery. To explain the sunspot cycle, in which the count of dark
sunspots and solar storminess peak at intervals of about 11 years, theorists
visualize a dynamo inside the Sun. Relative motions between neighbouring
layers of electrified gas supposedly drive the dynamo. As the years pass,
so the theory goes, the magnetic field becomes too strong for the gas to
hold, and it breaks out to the solar surface, causing sunspots and magnetic
explosions. The changes now observed are at the right depth for a dynamo.
"We are excited to see
the first evidence of changes close to the location of the solar dynamo,"
says the lead author, Rachel Howe of the National Solar Observatory in
Tucson, Arizona. "It's very surprising to find that the changes have such
a short period - 16 months or so rather than the 11 years of the solar
The flows of gas under study
occur about 220,000 km beneath the visible surface, or almost a third of
the way down to the centre of the Sun. Here is the supposed dynamo region
(tachocline) where the turbulent outer region, the convective zone, meets
the orderly interior, or radiative zone. The speed of the gas changes abruptly.
Near the equator the outer gas travels around the Sun's axis of rotation
faster than the inner gas. At mid-latitudes and near the poles, the outer
gas rotates more slowly.
The news from SOHO's MDI
and from GONG is that the contrast in speed between layers above and below
the supposed dynamo region can change by 20 per cent in six months. When
the lower gas speeds up, the upper gas slows down, and vice versa. In observations
spanning 4.5 years, from May 1995 to November 1999, these alternations
in speed occurred three times. They indicate a heartbeat of the Sun at
one pulse per 15-16 months in equatorial regions, and perhaps faster at
Revelation of what goes on
beneath the bright surface comes from helioseismology, which analyses motions
of the surface due to sound waves reverberating through the Sun's interior.
MDI, the Michelson Doppler Interferometer on the SOHO spacecraft, measures
the motions at a million points across the visible surface. Similar instruments
on the ground, at cloud-free sites in California, Hawaii, Australia, India,
Tenerife and Chile, make up the US-led Global Oscillation Network Group,
or GONG, and between them they observe the Sun for 24 hours a day. The
same pulses are seen in the space-based and the ground-based observations,
interpreted by two different analytical methods, which removes any doubt
about the reality of the phenomenon.
A European pioneer of helioseismic
theory, Jørgen Christensen-Dalsgaard of Aarhus University, Denmark,
participates in both the space and the ground projects. He is one of the
co-authors of the report in Science.
"For the very first time
we have a handle on important variations deep inside the Sun that are probably
linked to the sunspot cycle," Christensen-Dalsgaard comments. "Until
now, our attempts to explain the complicated magnetic changes seen at the
surface were pure speculation. One day, we may even be able to use observations
of the interior to predict the long-term changes in the Sun's outward behaviour
that affect the Earth."
The Science report also raises
the question of whether there may be a link between the deep changes and
another remarkable phenomenon seen by helioseismologists nearer the surface.
At depths down to 60,000 km, bands of gas parallel to the equator move
slightly faster or slower than the average speed for their solar latitudes.
Although the effect is subtle, it is very persistent, and the scientists
see the bands of fast and slow gas gradually moving from high latitudes
towards the equator, as the years go by. A similar equator-ward shift has
long been observed in the locations of sunspots, as the sunspot cycle approaches
its maximum of activity.
The solar heartbeat is the
second landmark discovery within a month, from SOHO's MDI instrument. On
9 March came the news that it had used sound waves to see right through
the Sun and detect sunspots on the far side. A Californian team led by
Philip Scherrer of Stanford University provided MDI, which is the largest
of three helioseismic instruments on SOHO. The latest results follow a
long series of achievements with MDI, including the detection of sub-surface
jet streams and the charting of the bubbling action under the ordinary
visible surface and under sunspots.
SOHO is a project of international
cooperation between ESA and NASA. The spacecraft was built in Europe for
ESA and equipped with instruments by teams of scientists in Europe and
the USA. NASA launched SOHO in December 1995, and in 1998 ESA and NASA
decided to prolong its highly successful operations until 2003. As a result,
the SOHO helioseismologists will be able to see whether the solar heartbeat
changes during the decline in activity after the sunspot maximum occurring