Nr. 29-99 - Paris, 8 July 1999
Solar scientists believe they may have solved yet another long-standing
enigma about the Sun. Working on data first gathered from the ESA's Solar
and Heliospheric Observatory (SOHO) and then by NASA's Spartan 201 spacecraft,
researchers have found that the solar wind streams out of the Sun by "surfing"
waves in the Sun's atmosphere.
The fact that this electrified plasma speeds up to almost 3 million
kilometres per hour as it leaves the Sun - twice as fast as originally
predicted - has been known for years.
The interpretation of how it happens is the real and surprising novelty:
"The waves in the Sun's atmosphere are produced by vibrating solar magnetic
field lines, which give solar wind particles a push just like an ocean
wave gives a surfer a ride" said Dr John Kohl, principal investigator
for the Ultraviolet Coronal Spectrometer (UVCS) - the instrument among
the 12 aboard SOHO which gathered the data - and for the Spartan 201 mission.
The outermost solar atmosphere, or corona, is only seen from Earth during
a total eclipse of the Sun, when it appears as a shimmering, white veil
surrounding the black lunar disc. The corona is an extremely tenuous, electrically
charged gas, known as plasma, that flows throughout the solar system as
the solar wind. The waves are formed by rapidly vibrating magnetic fields
in the coronal plasma. They are called magneto - hydro - dynamic (MHD)
waves and are believed to accelerate the solar wind.
The solar wind is made up of electrons and ions, electrically charged
atoms that have lost electrons. The electric charge of the solar wind particles
forces them to travel along invisible lines of magnetic force in the corona.
The particles spiral around the magnetic field lines as they rush into
space.
"The magnetic field acts like a violin string: when it's touched,
it vibrates. When the Sun's magnetic field vibrates with a frequency equal
to that of the particle spiraling around the magnetic field, it heats it
up, producing a force that accelerates the particle upward and away from
the Sun," says Dr. Ester Antonucci, an astronomer at the observatory
of Turin, Italy, and co-investigator for SOHO's UVCS an instrument developed
with considerable financial support by the Italian Space Agency, ASI.
In a way this is similar to what happens if two people hold a string
at opposite ends after threading it through an object, like a ring. If
one person wiggles the string rapidly up and down, waves form in the string
that move toward the person at the other end. The ring will "surf" these
waves and move toward the other person as well. Try it!
"Even with this major discovery, there are questions left to answer.
The observations have made it abundantly clear that heavy particles like
oxygen 'surf' on the waves, and there is also mounting evidence that waves
are responsible for accelerating the hydrogen atoms, the most common constituent
of the solar wind. Future observations are needed to establish this fact.
Many other kinds of particles, such as helium (second most common) have
never been observed in the accelerating part of the corona, and new observations
are also needed to refine our understanding of how the waves interact with
the solar wind as a whole," said Dr. Steven Cranmer of the Harvard-Smithsonian
Center for Astrophysics, lead author of the research to be published in
the Astrophysical Journal*.
Nevertheless, SOHO has again been able to reveal another of the Sun?s
mysteries: "This is another triumph for SOHO, stealing a long-held secret
from our Sun", said Dr Martin Huber, Head of ESA Space Science Department
and co-investigator for UVCS.
*Ref. Article by S.Cranmer, G.B. Field and J.L. Kohl on Astrophysical Journal ( June 20, Vol 518, p. 937-947) available on the web at:
http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v518n2/39802/sc0.html
Contacts:
Dr Martin Huber, Head of ESA Space Science Department
Co-investigator for UVCS
ESA/Estec
Postbus 299, 2200 AG Noordwijk zh, The Netherlands
Tel: +31 71 565 3552
Fax: +31 71 565 4699
Email: mhuber@estec.esa.nl
Dr Bernhard Fleck, ESA SOHO Project Scientist
NASA/Goddard Space Flight Center
Greenbelt, MD 20771, USA
Tel: (1) 301 286 4098
e-mail: bfleck@esa.nascom.nasa.gov
Dr Ester Antonucci
Osservatorio Astronomico di Torino
Strada dell? Osservatorio, 20, I-10025 Pino Torinese (TO), Italy
Tel: +39 011 8101913
Fax: +39 011 841281
Email: antonucci@astro.to.it
Dr. John L. Kohl, Principal investigator for UVCS
Harvard-Smithsonian Center for Astrophysics (CfA)
60 Garden Street, Cambridge, Massachusetts 02138 U.S.A.
Tel: (1) 617 495 7377
Fax: (1) 617 495 4984
Email: jkohl@cfa.harvard.edu
Dr Steven Cranmer
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Cambridge, Massachusetts 02138 U.S.A.
Tel: (1) 617 495 7271
Fax: (1) 617 495 7455
Email: scranmer@cfa.harvard.edu
For more information, please contact :
ESA Public Relations Division
Tel: +33 (0)1 53 69 7155
Fax: +33 (0)1 53 69 7690
Additional information on SOHO and the ESA science programme can be
found on the Worldwide Web at: http://sci.esa.int/
NOTES TO EDITORS: