Solar Smoke Rings
The Sun put on a dynamic show this week
with a series of swirling coronal mass ejections.
February 3, 2000 -- In J.R.R. Tolkien's well-known fantasy The
Hobbit, the diminutive inhabitants of the Shire loved to smoke
pipe-weed and blow intricate rings to delight their visitors.
Only the powerful wizard Gandalf could best a Hobbit in that
difficult art.
Beginning
on January 28, 2000, SOHO recorded a series of dramatic solar
coronal mass ejections (CMEs) as the Sun belched billions of
tons of hot gas into interplanetary space. One of them,
sighted on January 31, displayed swirling loops reminiscent of
Bilbo Baggins's most elegant smoke rings.
Above and to the right: These
coronagraph images captured by SOHO
on January 31, 2000, show a beautiful coronal mass ejection
erupting from the southwest limb of the Sun. The red-colored
animation, which targets the Sun's inner corona from 1.1 to 3
solar radii, spans a six hour period beginning at 0154 UT. The
swirls inside the CME are about 30 times bigger than Earth!
The blue image displays the outer corona from 1.5 to 30 solar
radii. It shows the CME at 1154 UT, about 10 hours after the
eruption began. For more information about SOHO coronagraphs, click
here.
Coronal mass ejections can carry up to 10
billion tons of plasma traveling at speeds as high as 2000
km/s. When they collide directly with Earth they can excite
geomagnetic storms, which have been linked to satellite
communication failures. In extreme cases, such storms can
induce electric currents in the Earth and oceans that can
interfere with or even damage electric power transmission
equipment.
Fortunately, none of the latest CMEs were
headed in our direction. Earth-directed mass ejections produce
what astronomers call "Halo
events." As they loom larger and larger they appear
to envelope the Sun itself.
The recent batch of CMEs -- mostly seen in
profile -- look very much like expanding tangled loops. While
there is some visual similarity to smoke rings, the physics of
a CME is different. Common down-to-Earth smoke
rings are spinning doughnut-shaped bundles of unmagnetized,
neutral gas. They have lower pressure on the inside than the
outside, and can travel considerable distances as they expand
and dissipate into the surrounding air. On the other hand, gas
inside a coronal mass ejection is completely ionized and
permeated by strong magnetic fields. The magnetic forces
dominate the movements and structure of the plasma. Most of
the loops seen in a CME are hot glowing gas trapped inside
curved magnetic fields.
If not for the coronagraphs on the Solar and Heliospheric
Observatory, many CMEs would never be noticed. At present,
SOHO is only spacecraft that monitors the Sun's outer corona
nearly around the clock. Two others, TRACE
and Yohkoh,
keep an eye on the inner corona, but CMEs are harder to spot
there. Space weather forecasters rely on SOHO for early
warnings of many Earth-directed solar eruptions, a service
that is increasingly important as we approach the solar
maximum in mid-2000.
"During solar maximum we often have
more than one coronal mass ejection every day," explains
David Hathaway, a solar physicist at the NASA Marshall Space
Flight Center. "The basic cause of CMEs is fairly well
understood. Like solar
flares, they occur whenever there's a rapid, large-scale
change in the sun's magnetic field. Solar flares and CMEs
often occur together, as they did this weekend, but not
necessarily because the flare triggers the CME or vice versa.
One can happen without the other and frequently during solar
maximum we see CMEs without an associated flare."
The CME on January 28, pictured here, was associated with a
weak C1.1-class solar flare near sunspot group 8848. That
active region had 3 tiny sunspots and a relatively
uncomplicated beta-type
magnetic field structure. C-class flares are ones that
register between 10-6 and 10-5 Watts per square meter in the 1
to 8 Angstrom X-ray band on NOAA's Earth-orbiting GOES 8
satellite. They are considered to be small compared to the
much larger M-class
and X-class flares that can erupt from active regions with
more complicated magnetic fields.
The "Bilbo Baggins" CME on January
31 was located near a larger sunspot group (8841) composed of
8 tiny spots. That active region also manifested a beta-type
field, but the NOAA Space Environment Center did not report an
associated solar flare. The January 31 CME is classified as a
"disappearing solar filament" (DSF) type, which
means that it occurred at about the same time that a filament
rose off the Sun's surface. Filaments are dense clouds of
material suspended above the surface of the Sun by loops of
magnetic field. They can remain in a quiescent state for
weeks. As the magnetic loops that support them slowly change,
they can erupt and rise off of the Sun in just a few minutes.
A filament viewed in profile over the limb of the sun is
called a prominence.
"Although we understand the basics of
why CMEs happen," continued Hathaway, "the details
are still unclear. What makes the fields unstable? How rapid
is the onset of the explosion? What's the detailed
relationship between flares and filaments and CMEs? All these
questions are being actively researched, and we still can't
predict CME events with any reasonable degree of
accuracy."
With solar maximum slated for mid-2000 solar
observers should have plenty of opportunity to study solar
flares and CMEs, and to hone their space weather forecasting
skills.
For more information about space weather and
current solar activity, please see SpaceWeather.com.
Technical information about current space weather condition
may be found at the NOAA
Space Environment Center. SOHO (the Solar and Heliospheric
Observatory) is a mission of international cooperation between
NASA and the European Space Agency. It is managed by the
Goddard Space Flight Center for the NASA HQ office of Space
Science.
Related Links:
Solar
Cinema - Cool movies of a recent solar prominence.
Make
Your Own Smoke Rings - tobacco free!
SpaceWeather.com
-follow the latest events on the Sun
Coronal
Mass Ejections -from the Marshall Space Flight Center
SOHO
home page -real-time images, screen savers, and more
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