Explosive Coronal Mass Ejections from the Sun

and their Propagation into the Interplanetary Medium.

The Sun as you have never seen it before

(Washington, DC) -- An international group of space scientists, using two experiments on the European Space Agency-NASA Solar Heliospheric (SOHO) satellite have produced a series of movies that show the origin and propagation of large solar explosions as never seen before. These explosions called Coronal Mass Ejections or CMES, if directed toward the Earth, cause large geomagnetic storms. The group believes, that the observations will vastly improve the understanding of the physical mechanism of CMEs and the solar corona.

In late December 1996, the group obtained a motion picture of the Sun as it sailed in front of the stars of the Sagittarius constellation and the Milky Way, while blowing its solar wind outwards in all directions around it. In the movie, the Sun is seen "swallowing" a comet that is known to the scientific team as Comet SOHO-6. Several coronal mass ejections are seen as visible puffs of gas.

Two instruments onboard the SOHO satellite contributed to the observation: The Large Angle Spectrometric Coronagraph (LASCO) instrument was built by a group of American, German, French and British scientists under the direction of Dr.Guenter Brueckner of the Naval Research Laboratory (NRL). A group of French, Belgian and American scientists under the direction of Dr. Jean Pierre Delaboudiniere from the Institute d'Astronomie Spatiale in France built the Extreme Ultraviolet Imaging Telescope (EIT).

Four images from the EIT instrument, taken in the ultraviolet coronal line of Fe XII, are shown in Fig. 1. Cool material (dark) ascends from the solar surface (20:32 universal time (UT)) to form a loop-like structure at 20:41 UT, which explodes, reaching an altitude of 300,000 km in 12 minutes. At 20:53 UT, the exploding material has accelerated to a velocity of 400 km/second. The cool material pushes in front of it a coronal (hot) shockwave, which can be seen as a bright circle. The shockwave was also recorded by the inner coronagraph of the LASCO instrument (Fig.2) at 21:04 and 21:07 in two different wavebands at an altitude of 450,000 km.

The explosion of the filament, together with the shockwave, results in the expulsion of a large amount of material from the overlying corona (Fig.3). Fig.3. shows "difference images". The previous image has been subtracted from the next. Therefore, an outward moving cloud shows as bright, while leaving in its wake a dark feature. The CME moves away from the Sun with a velocity of 450 km/second.

A frame of the "December 1996 movie" obtained with the SOHO/LASCO outer coronagraph C3 is shown in Fig.4.The field of view of this instrument encompasses 32 diameters of the Sun. To put this in perspective, the diameter of the images is 45 million kilometers (28 million miles) at the distance of the Sun, or half of the diameter of the orbit of Mercury. The Sun is located in the constellation Sagittarius. The center of the Milky Way is visible, as well as the dark interstellar dust rift, which stretches from the south to the north. The cloudy solar wind can be seen along the ecliptic plane, both over the east and the west limb of the Sun, stronger over the latter. The coronal mass ejection has now stretched out over the whole field of view to a distance of 22.5 million kilometers.

A frame of another movie, obtained with the SOHO-LASCO coronagraph C2 at the same time, is shown in Fig.5. The field of view of this coronagraph encompasses 8.4 million kilometers (5.25 million miles) of the inner heliosphere. The frame was selected to show Comet SOHO-6 as its head enters the equatorial solar wind region. It eventually plunged into the Sun. This comet entered the field of view of the C3 coronagraph on December 22, 1996, and disappeared on December 23 behind the occulter of the C2 coronagraph. (Coronagraphs are special telescopes with an external occultor, which blocks the glaring light of the solar disk from their extremely sensitive optics). Fig.5 also shows the inner streamer belt along the Sun's equator, where the low latitude solar wind originates and is accelerated. Over the polar regions, one sees the polar plumes all the way out to the edge of the field of view.

A later LASCO image, from January 6, 1997, revealed a large mass ejection directed toward 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 analyzer CELIAS detected an acceleration in the solar wind, from 350 to more than 500 kilometers 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 or may not have been a coincidence.

Mass ejections and other upheavals on the Sun will become even more common during the coming years, as the count of sunspots increases toward the expected maximum of solar activity in 2000 - 2001. Meanwhile, SOHO scientists are seeking the fundamental reasons for the cycle of sunspot activity, which is essentially a magnetic phenomenon.

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 two years of their scientific operations will have been completed.

The movie, some still photographs and this text can be downloaded from the LASCO home page. The URL is http://lasco-www.nrl.navy.mil/lasco.html. Go to special events, AAAS AMSIE'97 press release.

The Naval Research Laboratory is the Department of the Navy's corporate laboratory.

NRL conducts a broad Program of scientific research, technology and advanced development. The

Laboratory, with a total complement of nearly 4, 000 personnel, is located in southwest

Washington, DC, with other major sites at the Stennis Space Center, MS; and Monterey, CA.


A New View of the Sun - Video Release

The Large Angle Spectrometric Coronagraph C-3 Movie of December 1996

(Washington, DC) -- The LASCO C-3 movie of December 1996 shows the Sun in late December, sailing in front of the stars of the Sagittarius constellation and the Milky Way, while blowing its solar wind outwards in all directions. In movie, the Sun is seen "swallowing" a comet that is known to the scientific team as Comet SOHO-6. Several coronal mass ejections are also seen as visible puffs of gas.

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.

The remarkable images come from the Solar and Heliospheric Observatory's (SOHO's) visible-light coronagraph, LASCO. It masks the intense rays from the Sun's disk in order to reveal the much fainter glow of the outer solar atmosphere, the corona. 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 kilometers from the Sun's surface. Many stars are brighter than the gas, and they create the background scene.

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 December 2 1995. 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.

Roger Bonnet, the European Space Agency's director of science, shares the enthusiasm for the December 1996 movie, "For the first time we see the Sun clearly among the stars, thanks to LASCO on SOHO," 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 spacecraft ever devoted to the study of the Sun."

Notes NRL principal investigator, Dr. Guenter Brueckner, "I have spent my life examining the Sun, but this movie is a special thrill. Momentarily, I forget the years of effort that went into creating LASCO and SOHO. Leaving 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 affects our environment in ways that we are only now beginning to understand."

Features of the motion picture:

North is at the top of the scene, which corresponds with the orientation of the Sun as seen 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 to the right (west) in relation to the stars, during the period of observations. As a result, the Sun's position appears to shift to the left (eastward) in front of the stars.

The Sun is moving from the Sagittarius constellation toward Capricornus. That the Sun was in these constellations in December has been known since ancient times, but by calculations only, because the Sun's own brightness prevented a direct view of the starfield, The LASCO images are achievable only from a vantage point in space because the blue glow of the Earth's atmosphere hides the stars during the day.

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 toward 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 toward the heart of the Milky Way Galaxy, which lies in the constellation 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 caused by dense clouds of interstellar dust, which absorb the visible light from the stars.

On December 22, a doomed comet, previously unknown, enters on the lower left of the image. Its path curves toward the Sun and on December 23, 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 toward the visible surface of the Sun, the comet must have evaporated in the Sun's atmosphere. It was one of a family of comets known as sungrazers, believed to be remnants of a large comet that broke up perhaps 900 years ago. Other much bigger fragments were visible from the ground 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 exact coordinates of these comets, which now in progress, are a prerequisite for their inclusion in the official record of comet discoveries.

Debris strewn from the tail of many comets make up a disk of dust around the Sun, in the ecliptic plane where the planets orbit. It scatters sunlight and it is sometimes visible at twilight on the Earth, as the Zodiacal Light. In the raw images obtained by LASCO, the Zodiacal Light is 50 times brighter than the solar wind at the edge of the field of view. Image processing has to subtract its effect 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 scenery. Cosmic rays are energetic particles coming from exploded stars in the universe. Variations in the interplanetary magnetic field influence their intensity in the vicinity of SOHO and the Earth. Operating beyond the Earth's magnetic field, which repels most of the cosmic ray particles, SOHO is more exposed to the cosmic rays.

In the largest outburst from the Sun seen in the December movie, a coronal mass ejection causes billions of tons of gas to race out into space on the right-hand (western side) of the Sun. The origin of this event, which was much lower in the Sun's atmosphere, was evident in an expanding bubble seen in processed images from EIT on board SOHO. (See also Fig. 1). Coronagraph views obtained during the same December periods in the narrower fields of LASCO's C I and C2 instruments also helped to reveal the Sun's complex behavior.

The Naval Research Laboratory is the Department of the Navy's corporate laboratory.

NRL conducts a broad program of scientific research, technology and advanced development. The

Laboratory, with a total complement of nearly 4, 000 personnel, is located in southwest

Washington, DC, with other major sites at the Stennis Space Center, MS,- and Monterey, CA.