Feb 2, 2000

Solar Smoke Rings

Coronal Mass Ejection on Jan 31, 2000
Coronal Mass Ejection on Jan 31, 2000
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.

Here in the real world, the best way to enjoy an enchanting tobacco-free smoke ring may surprise you. Simply tune in to the Solar and Heliospheric Observatory (SOHO), an Earth-orbiting observatory with an eye on the biggest puffer in the solar system -- the Sun.

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: 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.

SOHO image
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. (click for animation)

"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."

January 28 CME
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.

Parents and Educators: Please visit Thursday's Classroom for lesson plans and activities related to this story.

For more information about space weather and current solar activity, please see 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.

Web Links

Solar Cinema - Cool movies of a recent solar prominence.

Make Your Own Smoke Rings - tobacco free! -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