Trio peeks at secret recipe for stormy solar weather
Posted: June 21, 2003

A three-spacecraft collaboration recorded for the first time the entire initiation process of a high-speed eruption of electrified gas from the Sun, providing clues about the Sun's secret recipe for stormy weather. The April 21, 2002 observation confirmed the predominant scenario for how these eruptions, called Coronal Mass Ejections, are blasted from the Sun.

A false-color image sequence over two hours of a large coronal mass ejection March 20, 2000 taken by the LASCO C2 instrument. In this sequence a CME blasts into space a billion tons of particles traveling millions of miles an hour. This CME was not headed towards Earth, though the ones that are can cause problems with satellites, communications, and power equipment when the CME hits the Earth's magnetic field. Credit: NASA/ESA
The three spacecraft involved were NASA's Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), which takes pictures of flaring regions using the Sun's high-energy X- rays and gamma rays; NASA's Transition Region and Coronal Explorer (TRACE), which makes images using ultraviolet light from the Sun; and the Solar and Heliospheric Observatory (SOHO) spacecraft, a collaboration between NASA and the European Space Agency.

"This was the first time that we have been able to identify and study in detail the region on the Sun where the initiation and acceleration of a coronal mass ejection occurs," said Dr. Peter Gallagher, research scientist for RHESSI and SOHO at NASA's Goddard Space Flight Center, Greenbelt, Md., and lead author of two papers on this research. "We now have a better understanding of how the energy release above the surface of the Sun relates to the ejection of material, perhaps allowing some real-time forecasts." The results are being presented today during a meeting of the American Astronomical Society's Solar Physics Division in a press conference at the Johns Hopkins University Applied Physics Laboratory, Laurel, Md.

Coronal Mass Ejections (CME) are often associated with solar flares. A flare is a giant explosion in the solar atmosphere that spews radiation and results in the heating of solar gas and the acceleration of particles to nearly the speed of light. Both events can be initiated in a matter of seconds, making their joint observations difficult to coordinate.

The twisting and snapping of magnetic field lines on the Sun, called magnetic reconnection, seem to cause CMEs and solar flares. When these fields snap from the buildup of magnetic energy, plasma is heated and particles are accelerated, resulting in massive explosions and emitting radiation ranging from radio waves to X-rays.

Frequently, a CME and flare will burst from the same region of the Sun nearly simultaneously. Just like the debate over whether the chicken or the egg came first, solar researchers discuss whether flares cause CMEs or the reverse, or if they are more loosely associated.

The April 21, 2002 CME observed by several instruments onboard the SOHO spacecraft. The green image shows the Sun's inner atmosphere or corona, from which the CME emerged. The later stages of the expanding CME are visible in the red and blue colored images taken with LASCO. The CME is the large, wispy white cloud seen to the right of the red and blue images. White dots in the image result from particles from the flare hitting the cameras of the LASCO instrument. Credit: NASA/ESA
The April 21, 2002 observation confirmed the predominant scenario for high-speed CMEs (those moving at one million to 5 million miles per hour or 1.6 million to 8 million km/hr.). This is where solar magnetic fields act like a lid, holding down a blob of gas (CME) that is trying to rise. Somehow, the magnetic lid opens, possibly as a result of magnetic reconnection and the generation of a flare, and then the CME rises from the Sun, dragging the magnetic fields with it. Magnetic reconnection continues to energize the associated flare for over 12 hours.

All three spacecraft played vital roles in confirming that this was the process. First, RHESSI saw a gradually increasing burst of X-rays announcing the start of the flare. TRACE observed the CME in the extreme ultraviolet as it began to rise from the Sun. Several minutes later, RHESSI saw a burst of high energy X-rays under the erupting CME, and TRACE saw a similar explosion of ultraviolet rays, both indicating a large flare. SOHO then captured the CME as it continued moving away from the Sun.

"Each of these spacecraft is quite complementary," said Gallagher. "It's only through their coordination in this observation that we're now able to understand the predominant scenario for these fast, large coronal mass ejections and the associated flares."

The current results feed into the decades-old controversy over whether solar flares cause coronal mass ejections, or vice versa. While the first signs of the flare occur before the CME liftoff, the bulk of the flare energy is released later, after the CME has already been accelerated. The two phenomena are revealed to be merely different aspects of the same event, according to the team.

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