The Sun as You Have Never Seen it Before - LASCO

2/14/1997 - 10-97r
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Note: The movie, some still photographs and this text can be downloaded from the LASCO home page. The URL is Go to special events, AAAS AMSIE'97 press release.

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 occultor 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.

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