ESA Science & Technology - INFO 06-1997: The Sun as you never saw it before

The remarkable images come from SOHO's visible-light coronagraph LASCO. It masks the intense rays from the Sun's surface in order to reveal the much fainter glow of the solar atmosphere, or corona. Operated with its widest field of view, in its C3 instrument, LASCO's unprecedented sensitivity enables it to see the thin ionized gas of the solar wind out to the edges of the picture, 22 million kilometres from the Sun's surface. Many stars are brighter than the gas, and they create the background scene.

Coronal Mass Ejection

The results alter human perceptions of the Sun. Nearly 30 years ago, Apollo photographs of the Earth persuaded everyone of what until then they knew only in theory, that we live on a small planet. Similarly the new imagery shows our motion in orbit around the Sun, and depicts it as one star among - yet close enough to fill the sky emanations that engulf us.

For many centuries even astrologers knew that the Sun was in Sagittarius in December and drifting towards the next zodiacal constellation, Capricornus. This was a matter of calculation only, because the Sun's own brightness prevented a direct view of the starfield. The SOHO-LASCO movie makes this elementary point of astronomy a matter of direct observation for the first time.

A special allocation of observing time, and of data tranmission from the SOHO spacecraft, enabled the LASCO team to obtain large numbers of images over the period 22-28 December 1996. Since then, a sustained effort in image processing, frame by frame, has achieved a result of high technical and aesthetic quality. Only now is the leader of the LASCO team, Guenter Brueckner of the US Naval Research Laboratory, satisfied with the product and ready to authorize its release.

"I spend my life examining the Sun," Brueckner says, "but this movie is a special thrill. For a moment I forget the years of effort that went into creating LASCO and SOHO, and leave aside the many points of scientific importance in the images, I am happy to marvel at a new impression of the busy star that gives us life, and which affects our environment in many ways that we are only now beginning to understand."

Transatlantic Cooperation

The Solar and Heliospheric Observatory SOHO is a project of international cooperation between ESA and NASA. ESA and the European aerospace industry built the spacecraft, and NASA launched it on 2 December 1995. Operating 1.5 million kilometres out on the sunward side of the Earth, near the position called Lagrangian point L1, SOHO has an uninterrupted view of the Sun from an undisturbed vantage point, and a precision of pointing which makes delicate observations possible. SOHO carries 12 sets of instruments provided by scientific teams, each led by a European or an American principal investigator. They study the solar interior by helioseismology, the solar atmosphere seen by ultraviolet and visible light, and the solar wind and energetic particles.

There is much transatlantic collaboration within the various teams. Besides the Naval Research Laboratory in Washington, LASCO involves the Max-Planck-Institüt für Aeronomie at Lindau (Germany), the Unversity of Birmingham (England) and Laboratoire d'Astronomie Spatiale at Marseille (France). Sharing LASCO's electronic systems, and many operations and analyses, is SOHO's extreme ultraviolet imager EIT. This is the responsibility of a team led from Orsay (France) and it observes activity in the Sun's hot atmosphere related to the wider events seen by LASCO.

Roger Bonnet, who presides over the multinational effort as ESA's Director of Science, shares the enthusiasm for the Christmas movie. "For the first time we see the Sun clearly among the stars, thanks to SOHO and LASCO," Bonnet comments. "Now when we say that the Sun is a typical star, and a key to understanding the whole Universe, that is no longer a theoretical statement but something everyone can see. The quality of the images confirms that SOHO is the finest and most stable spacecraft ever devoted to the study of the Sun."

Features of the Motion Picture

North is at the top of the scene, which corresponds with the orientation of the Sun as seen at midday in the northern hemisphere of the Earth. SOHO's progress in orbit around the Sun remains in step with the Earth's motion. It travels towards the right (west) in relation to the stars, during the period of observation. As a result, the Sun's position appears to shift to the left (eastwards) in front of the stars.

LASCO C3 observes an area of the sky 32 times wider than the visible Sun itself. If you spread the fingers of one hand and hold them at arm's length towards the sky, they will span the 17-degree width of LASCO's field of view. For comparison, the Sun is less than half the width of your little finger.

At the time of the observations, SOHO is looking towards the heart of the Milky Way Galaxy, which lies in the constellation of Sagittarius. The Milky Way, made by the light of billions of distant stars, forms a luminous band slanting down and to the right. Dark lanes seen in the Milky Way are real features familiar to astronomers. They are created by dust clouds in the disk of the Galaxy which obscure the distant stars.

A doomed comet, previously unknown, enters on the left of the image on 22 December. Its path curves towards the Sun and on 23 December. Its path curves towards the Sun and on 23 December it disappears behind the occulting mask of the coronagraph. It fails to reappear on the far side of the Sun. Whether or not its trajectory took it directly towards the visible surface, the comet must have evaporated in Sun's atmosphere. It was one of a family of comets known as sungrazers, believed to be remnants of a large comet that that broke up perhaps 900 years ago. Other fragments were responsible for spectacular comet apparitions in 1843, 1882 and 1965.

The object in the movie is called Comet SOHO 6. It is one of seven sungrazers discovered so far by LASCO, with its unparalleled view of the solar vicinity. Analyses of the comets'orbits, now in progress, are a prerequisite for their inclusion in the official record of comet discoveries. LASCO also provided unique pictures of Comet Hyakutake passing behind the Sun at the beginning of May 1996.

Debris strewn from the tails of many comets makes a disk of dust around the Sun, in the ecliptic plane where the planets orbit. It scatters sunlight and is sometimes visible at twilight on the Earth, as the Zodiacal Light. In the raw images obtained by LASCO, the Zodiacal Light is brighter than the solar corona. Image processing has to subtract its effects precisely, to bring the solar wind and the Milky Way into plain view.

Random flashes of light in the images are due to cosmic rays striking the detector. These should be regarded, not as blemishes, but as part of the scenery. Cosmic rays are energetic particles coming from exploded stars in the Milky Way, and variations in the solar wind influence their intensity in the vicinity of SOHO and the Earth. Operating beyond the Earth's magnetic field, which repels many particles, SOHO is more exposed to the cosmic rays.

In the largest outburst from the Sun seen in the Christmas movie, a mass ejection causes billions of tonnes of gas to race out into space on the right-hand (western) side of the Sun. The origin of this event much lower in the Sun's atmosphere was evident in an expanding bubble seen in processed images from the extreme ultraviolet imager EIT. Coronagraph views obtained during the same Christmas period in the narrower fields of LASCO's C1 and C2 instruments also helped to reveal the Sun's complex behaviour.

Coronal mass ejections are the hurricanes of space weather. SOHO is ideally placed and instrumented to report and even anticipate their origins in the Sun's atmosphere. Although the Sun is supposedly very quiet at present, being close to the minimum count of sunspots, LASCO observes so many outbursts large and small - roughly one a day - that scientists are having to think again about how to define a coronal mass ejection.

SOHO's Continuing Success

Later LASCO images, on 6 January 1997, revealed a large mass ejection directed towards the Earth. As it swelled it appeared as a halo around the Sun. The mass ejection reached SOHO itself less than four days later, and the solar-wind analyser CELIAS detected an acceleration in the solar wind, from 350 to more than 500 kilometres per second. Soon afterwards, American, Russian and Japanese satellites operating closer to the Earth registered the event, which caused a magnetic storm and bright auroras. The failure of an American TV satellite on 11 January may have been directly related to this event.

Mass ejections and other upheavals on the Sun will become even commoner during the coming years, as the count of sunspots increases towards the expected maximum of solar activity in 2000-01. Meanwhile, SOHO is seeking the fundamental reason for the cycle of sunspot activity, which is essentially a magnetic phenomenon.

One of the helioseismic instruments probing the solar interior, SOI/MDI, has detected a likely source for the Sun's puzzling magnetism. There may be a natural dynamo operating at the base of the turbulent outer region of the Sun, called the convective zone. This rotates about 7 per cent faster than the underlying and more cohesive region of dense gas, the radiative zone.

With the spacecraft in excellent condition and their instruments performing beyond expectations, SOHO's scientists are urging ESA and NASA to allow them to continue their work beyond April 1998, when the initial year of their scientific operations will have been completed.