A sunspot turns out to be a kind of whirlpool, where hot gas near the
Sun’s surface converges and dives into the interior at speeds of up to
4000 kilometres per hour. This is the latest discovery by the ESA-NASA
SOHO spacecraft. Solar physicists have long known that intense magnetic
fields in sunspots strangle the normal upflow of energy from the interior,
leaving the sunspot cooler and therefore darker than its surroundings. The
converging flows of gas around a spot, found by SOHO, explain why the
magnetic fields become concentrated, and how a sunspot can persist for
days or weeks.

Bernhard Fleck, ESA’s project scientist for SOHO, comments, “The origin
and stability of sunspots has been one of the long-standing mysteries in
solar physics. I am delighted to see that with SOHO we are beginning to
crack this problem.”

The gas flows around and beneath a sunspot have been detected by a team of
scientists in the USA, using the Michelsen Doppler Imager (MDI) on SOHO.
The instrument explores the solar interior by detecting natural sound
waves at a million points on the Sun’s surface.

“After many years of contradictory theories about sunspots, MDI on SOHO is
at last telling us what really happens,” comments Junwei Zhao of Stanford
University, California, lead author of a report published in the
Astrophysical Journal. Inflows and downflows similar to those now detected
with SOHO were envisaged in 1974 by Friedrich Meyer of Germany’s
Max-Planck-Institut für Physik und Astrophysik, and his colleagues. A
similar expectation figured in a theory of sunspots advanced in 1979 by
Eugene Parker of Chicago. “Our observation seems to provide strong
evidence for both predictions,” Zhao says.

Sunspots have fascinated scientists since Galileo’s time, 400 years ago,
when they shattered a belief that the Sun was divinely free of any
blemish. As symptoms of intense magnetic activity, sunspots are often
associated with solar flares and mass ejections that affect space weather
and the Earth itself. The Sun’s activity peaks roughly every 11 years, and
the latest maximum in the sunspot count occurred in 2000.

Even with huge advances in helioseismology, which deduces layers and flows
inside the Sun by analysis of sound waves that travel through it and
agitate the surface, seeing behind the scenes in sunspots was never going
to be easy. The MDI team refined a method of measuring the travel time of
sound waves, invented in 1993 by Thomas Duvall of NASA Goddard, called
solar tomography. It is like deducing what obstacles cross-country runners
have faced, just by seeing in what order the contestants arrive at the
finish. Here the runners are packets of sound waves, and the obstacles are
local variations in temperature, magnetic fields and gas flows beneath the
Sun’s surface.

“We needed better mathematical tricks,” comments Duvall. “So we put
together ideas from classical and quantum physics, and also from a recent
advance in seismology on the Earth.”

In an earlier application of solar tomography, the team examined in detail
the ante-natal events for an important group of sunspots born on 12
January 1998. They found sound waves beginning to travel faster and faster
through the region where sunspots were about to form. Less than half a day
elapsed between signs of unusual magnetic activity in the Sun’s interior
and the appearance of the dark spots on a previously unblemished surface.

“Sunspots form when intense magnetic fields break through the visible
surface,” says Alexander Kosovichev of Stanford. “We could see the
magnetic field shooting upwards like a fountain, faster than we expected.”

Even late on the previous day there was little hint of anything afoot,
either at the surface or in the interior. By midnight (Universal Time) a
region of strong magnetic field had risen from a depth of 18 000
kilometres and was already half way to the surface, travelling at 4500
km/hr. Sound speeds were increasing above the perturbed zone. By 8:00 a.m.
an intense, rope-like magnetic field was in possession of a column of gas
20 000 kilometres wide and reaching almost to the visible surface. In the
uppermost layer beneath the surface, the magnetic rope divided itself into
strands that made the individual sunspots of the group.

Under a large, well-established sunspot, in June 1998, the sound waves
revealed a persistent column of hot, magnetised gas rising from deep in
the interior. At a depth of 4000 kilometres it spread fingers towards
neighbouring parts of the surface where it sustained some smaller
sunspots. The magnetic column was not connected to another nearby spot
where the magnetic field went in the opposite direction. Immediately below
the large spot was a cushion of cooler, less intensely magnetised gas.

A closer look at the gas flows, during the development of that June 1998
sunspot, led to the further findings now reported. The inflows and
downflows in the immediate vicinity of the sunspot reach downwards for
only a few thousand kilometres from the surface, which means less than one
per cent of the distance to the Sun’s centre. The discovery therefore
depended on MDI’s unique ability to explore just below the surface.

The whirlpool of gas is responsible for the persistence of a sunspot. The
cooling due to the magnetic field of the sunspot provokes the down-flow,
and the gas disappearing downwards is replaced by more gas flowing inwards
towards the spot. It brings with it its own associated magnetic field and
prevents the strong magnetic field of the sunspot from dissipating. So the
cooling and downflow continue, and the process is self-sustaining.

The downflow of gas may also help to explain the puzzling fact that the
Sun is actually brighter when it is freckled with dark spots. The VIRGO
instrument on SOHO, operated by a Swiss-led team, confirmed the
observations of earlier solar spacecraft, showing that sunshine is
slightly more intense at sunspot maximum. Douglas Gough of Cambridge
University, a leading solar theorist, notes that the downflow of gas seen
by MDI on SOHO can redistribute energy bottled up by a sunspot.

“What is interesting from the physical point of view is that, being cool,
the descending flow is readily able to extract the heat that accumulates
beneath the spot,” Gough says. “It then spreads the heat away from the
sunspot and eventually brings it to the surface of the Sun far from the
spot, from where it is radiated into space.”

Note to editors

The SOHO project is an 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 extend its highly successful
operations until 2003.

For more information please contact:

ESA – Communication Department, Media Relations Office

Tel: +33(0)

Fax: +33(0)

Dr. Bernhard Fleck, ESA – SOHO Project Scientist

ESA Space Science Dept, c/o NASA- GSFC, Greenbelt (Maryland USA)

Tel: +1 301 286 4098

Fax: +1 301 286 0264

Email: bfleck@esa.nascom.nasa.gov

Note on scientific papers

‘Investigation of Mass Flows Beneath a Sunspot by Time-Distance
Helioseismology’ by Junwei Zhao and Alexander G. Kosovichev (Stanford) and
Thomas L. Duvall Jr (NASA Goddard) is published in Astrophysical Journal,
vol. 557, p. 384, 2001. A related paper, ‘Time-Distance Inversion Methods
and Results’ by Kosovichev, Duvall and Philip H. Scherrer (Stanford)
appeared in Solar Physics, vol. 192, p. 159, 2000.

For more material about this story (images) and information about the
SOHO mission and the ESA Science Programme, visit the ESA Science Website
at http://sci.esa.int

Other useful links

SOHO Home page http://soho.estec.esa.nl

MDI home page: http://soi.stanford.edu/

MDI Sunspot results page: http://soi.Stanford.EDU/press/ssu09-01/

NASA press release: