Note: 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.
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.
The U.S. Naval Research Laboratory is the Navy's full-spectrum corporate laboratory, conducting a broadly based multidisciplinary program of scientific research and advanced technological development. The Laboratory, with a total complement of nearly 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 85 years and continues to meet the complex technological challenges of today's world. For more information, visit the NRL homepage or join the conversation on Twitter, Facebook, and YouTube.
Comment policy: We hope to receive submissions from all viewpoints, but we ask that all participants agree to the Department of Defense Social Media User Agreement. All comments are reviewed before being posted.