N° 29–1999: Solar winds surfs 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:

* SOHO is a cooperative mission between ESA and NASA. The spacecraft was launched from Cape Canaveral on 2 December 1995.

* Spartan 201 spacecraft last flown in October- November 1998 during shuttle mission STS-95 saw the participation of the European astronaut Pedro Duque besides the US astronaut John Glenn.

* At the speed of the fast solar wind -- 3 million kilometres per hour -- a spacecraft in low Earth orbit would be able to circle our planet 75 times in one hour. Normally, satellites take about 1.5 hours to make just one orbit.