A composite image from two instruments in the solar spacecraft SOHO gives a clear impression of different mechanisms at work in the solar atmosphere, creating two kinds of wind flowing outwards from the Sun. When the Solar and Heliospheric Observatory was launched, just a year ago this week, one of its main tasks was to identify the sources of the solar wind, which blows non-stop into the solar system and influences all the planets, including the Earth. This line of investigation is already full of promise.

In the equatorial regions of the Sun, SOHO's extreme ultraviolet imager EIT reveals frenzied activity in a hot atmosphere. It contrasts with more orderly conditions near the poles, in cooler regions called coronal holes. Another instrument in SOHO, the ultraviolet coronagraph UVCS, makes images of emissions from charged oxygen atoms high above the Sun's visible surface, where the generators of the solar wind are at work.

In the equatorial zone, the Sun's magnetic field tries to bottle up the superheated gas. The gas wins the fight and some of it bursts out in funnel-shaped features called helmets. This break-out creates the "slow" solar wind, at 350-400 kilometres per second, which drags the magnetic lines of force with it, far out into the solar system. It seems to be a heat-driven wind, as indicated by UVCS measurements that indicate lower speeds for heavy atoms such as oxygen, compared with the nimbler hydrogen atoms.

The wind generator over the coronal holes is completely different. Here the Sun's magnetic field offers no resistance to the outflow of material. Another mechanism accelerates the "fast" solar wind that blows from the coronal holes at 700-800 kilometres per second. It may involve high frequency magnetic waves. John Kohl of the Smithsonian Astrophysical Observatory (USA) and Giancarlo Noci of the University of Florence (Italy) believe that their instrument in SOHO will identify the fast wind generator.

"UVCS reveals an amazing state of affairs at a height of about 1,700,000 kilometres above the coronal holes," Kohl explains. "There our oxygen atoms are far more agitated than the hydrogen, with 60 times more energy of motion. They rush about as if they were scalded at 200 million degrees C. So we are homing in on the mechanism that accelerates the fast solar wind, with this very strong clue that it favours the heavier elements".

At greater distances from the Sun, SOHO's visible-light coronagraph LASCO traces the flow of the slow solar wind far into space, and sees it carrying intermittent bright patches corresponding with relatively dense concentrations of solar material. These gusts are milder than the occasional mass ejections also seen by LASCO, which accompany great convulsions in the solar magnetic field.

SOHO's solar wind analyser CELIAS has detected many previously unrecorded elements and isotopes among the charged atoms of the solar wind. The solar wind mapper, SWAN, observes the widespread effects of solar wind particles as they interact with the atoms of an interstellar breeze blowing into the solar system. Yet the results on the solar wind represent only a fraction of SOHO's achievements so far, with twelve sets of instruments observing everything from oscillations deep inside the Sun, to the solar influence on energetic cosmic rays coming from the Galaxy.

Stealing the show in helioseismology

SOHO is a project of international cooperation between the European Space Agency and NASA. The spacecraft was built in Europe and instrumented by scientists on both sides of the Atlantic. NASA launched SOHO and provides the ground stations and an operations centre at the Goddard Space Flight Center near Washington. SOHO has an uninterrupted view of the Sun from a halo orbit around Lagrangian Point N 1 where the gravity of the Sun and the Earth are in balance, 1,500,000 kilometres out on the sunward side of the Earth. The spacecraft's engineering has proved to be excellent and no practical difficulty is anticipated in keeping SOHO operational into the sunspot maximum expected in 2000-2001.

SOHO was launched on 2 December 1995. Check-out observations with some instruments began just a few days later. SOHO attained its L1 halo orbit on 14 February 1996, and commissioning was formally completed on 16 April. Already the first results were showing unprecedented images of the solar atmosphere, of the heliosphere filled by the solar wind, and even of the Sun's interior as revealed by oscillations due to sound waves in the helioseismic technique.

At a recent meeting in Nice (France) the world's helioseismologists were enthralled by SOHO's unprecedented images of shallow flows of material, just below the Sun's visible surface, which ground-based instruments have not been able to observe. These results came from the MDI instrument, which analyses oscillations at a million points on the Sun's visible surface. SOHO gives scientists a wholly new impression of the churning motions in the hot gas, and how these interact with concentrations of the Sun's magnetic field. SOHO's results are expected to steal the show again at a meeting on helioseismology in London this month (13 December).

Like MDI, the other helioseismic instruments in SOHO called GOLF and VIRGO benefit by the escape from the Earth's atmosphere. As SOHO discovered, turbulence in the air spoils the hi-fi recordings of the Sun's sound waves, with background noise. Prolonged observations from SOHO's unique undisturbed vantage point at L1 will achieve an extremely high precision of recording and will enable these instruments to provide crucial new information about the interior, extending all the way down to the Sun's power source, at its thermonuclear core.

Birthday greetings from leading experts

As the images and other data continue to pour from SOHO at a high rate, a revolution in solar science is in progress. Among many solar experts who are enthusiastic about SOHO's successes is the leading astrophysist Evry Schatzman of the Observatoire de Meudon.(France).

"On SOHO's first birthday", Schatzman says, "I congratulate my European and American colleagues on the most remarkable and successful spacecraft ever devoted to examining the star on which our lives depend. SOHO's astounding ability to probe the Sun's interior by helioseismology gives me hope that we shall at last solve the ancient mystery of the sunspots and the magnetic cycle. The observations of ultraviolet rays and energetic particles give us our best chance of understanding the hot atmosphere and its emissions into the solar system; But to fulfil its high promise, SOHO must continue operating at least until the maximum of sunspot activity around the year 2000".

Joining in SOHO's anniversary greetings is Eigil Friis Christensen, a solar-terrestrial physicist at the Danish Meteorological Institute. He has played a prominent role in tracing the effects of solar variations on the terrestrial climate.

"SOHO is now vital for understanding the Earth's environment", Friis Christensen says. "I am convinced that long-term changes in the strength and variability of the solar wind alter the climate, but no one knows why those changes occur. In the years ahead, as it follows the dramatic events leading from the sunspot minimum to the sunspot maximum, SOHO should reveal the processes inside the Sun that influence the character of the solar wind. If so, it will open a new chapter in solar-terrestrial climatology".

Other highlights from SOHO's programme

Previous ESA Information Notes (07-96 and 16-96) and photo releases have sketched SOHO's results on the solar wind and coronal mass ejections, mentioned here in connection with the LASCO, UVCS, CELIAS and SWAN instruments. The particle detectors COSTEP and ERNE have registered high-energy electrons and hydrogen and helium nuclei flung out by violent eruptions in the Sun. These particle events will become commoner as the sunspot count increases and the Sun becomes stormier.

The EIT imager's many observations of the solar atmosphere by extreme ultraviolet rays have included spectacular images of tangled magnetic fields creating intense disturbances even at times when the Sun appeared extremely calm to visible-light telescopes on the ground. SOHO's ultraviolet spectrographs SUMER and CDS analyse the never-ending explosive events in the atmosphere in more detail. This is part of a concerted effort to fulfil another of SOHO's key tasks, in explaining how the Sun's atmosphere attains temperatures of millions of degrees, in contrast with the temperature of less than 6000 degrees C of the visible surface.

Acronyms :

CDS : Coronal Diagnostic Spectrometer (PI : R. Harrison, RAL, England)

CELIAS : Charge, Element and Isotope Analysis System (PI : P. Bochsler, Bern University, Switzerland)

COSTEP : Comprehensive SupraThermal and Energetic Particle analyser (PI : H. Kunow, University of Kiel, Germany)

EIT : Extreme-ultraviolet Imaging Telescope (PI : J.P. Delaboudini[re, IAS Orsay, France)

ERNE : Energetic and Relativistic Nuclei and Electron experiment (PI : J. Torsti, University of Turku, Finland)

GOLF : Global Oscillations at Low Frequencies (PI: A. Gabriel, IAS Orsay, France)

LASCO : Large Angle Spectroscopic Coronagraph (PI: G. Brueckner, NRL Washington, USA)

MDI : Michelson Doppler Imager (PI : P. Scherrer, Stanford University, USA)

SUMER : Solar Ultraviolet Measurements of Emitted Radiation (PI : K. Wilhelm, MPAe Lindau, Germany)

SWAN : Solar Wind Anisotropies (PI : J.L. Bertaux, SA Verri[res le Buisson, France)

UVCS : UltraViolet Coronagraph Spectrometer (PI: J. Kohl, SAO, Cambridge, USA)

VIRGO : Variability of solar Irradiance and Gravity Oscillations (PI: C. Froehlich, PMOD/WRC Davos, Switzerland)

SOHO : Solar and Heliospheric Observatory

Figure: The Sun's outer atmosphere

Photo credits :

SOHO : (UVCS and EIT consortia) : ESA, NASA

UVCS : Smithsonian Astrophysical Observatory, University of Florence and Agenzia Spaziale Italiana (Italy)

EIT : Institut d'Astrophysique Spatiale, Orsay, France, Centre Spatial de Li[ge, Belgium, Naval Research Laboratory, Goddard Space Flight Center

SOHO is a project of international cooperation between ESA and NASA