| SPACECRAFT TRIO
PEEKS AT SECRET RECIPE FOR STORMY SOLAR WEATHER
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The Sun is a mighty chef. It stirs sunspots, boils
electrified gasses, then serves up the main course - violent
solar weather in the form of a coronal mass ejection (CME)
with a side of solar flare. Like superstar chefs everywhere,
the Sun carefully guards its recipes. However, three
spacecraft - RHESSI, SOHO and TRACE - have worked in
collaboration to record for the first time the entire
initiation process of a high-speed CME, providing clues about
the Sun's secret recipe for stormy weather and ultimately
helping scientists understand turbulent solar activity and its
effects on Earth.
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CMEs are billion-ton eruptions of electrically charged gas
(plasma) in the Sun's atmosphere (corona). The fastest CMEs
are blasted into space at a speed of up to five million miles
per hour (eight million km/hr.). CMEs 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.
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The three spacecraft involved were NASA's Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI), which takes
pictures of flaring regions on the Sun 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.
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"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."
If a CME is directed towards Earth, it can shut down
electrical systems and disrupt signals from satellites and
aircraft when it hits the magnetic field surrounding our
planet. Understanding how the Sun "heats and serves" a CME and
knowing what type of energy to look for prior to one will help
scientists predict when a CME will occur so they can issue
warnings.
The twisting and snapping of magnetic field lines on the
Sun, called magnetic reconnection, seem to cause CMEs and
solar flares, among other solar events. 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 and
gamma 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 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.
For movies and more images, refer to:
http://hesperia.gsfc.nasa.gov/~ptg/hessi/20020421/
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