Dark features resembling Earth-sized tadpoles were seen swimming in
the atmosphere of the Sun after it was heated to millions of degrees
following an enormous explosion, according to scientists who made the
observation using NASA’s Transition Region and Coronal Explorer
(TRACE) spacecraft.

“This is the best view yet of these enigmatic shapes,” said Dr.
Edward Deluca of the Harvard-Smithsonian Astrophysical Observatory
(SAO), Cambridge, Mass., who is a co-author of a paper on the
observation to be submitted to the Astrophysical Journal in September
2003. The observation is expected to shed light on the physics of
magnetic reconnection, the process believed to power solar
explosions, which occasionally disrupt satellites and power systems.
The result is presented today as a poster at the American Geophysical
Union meeting in Nice, France.

The explosion on April 21, 2002, was an “X-class” solar flare, the
most powerful kind, releasing about as much energy as a billion
one-megaton nuclear bombs. It was also associated with a coronal mass
ejection (CME), a multi-billion ton eruption of electrified gas
(plasma) into space.

The tadpoles are mysterious in part because of their behavior. “In
the vicinity of a solar flare associated with a CME, most matter is
moving away from the solar surface, but the tadpoles move downward at
initial speeds between 30 and 600 kilometers per second (about 19 to
373 miles/sec.), something you don’t expect,” said Dr. David McKenzie
of Montana State University – Bozeman, who has observed these
features many times before at lower resolution with the Soft X-ray
Telescope on board the Japanese Yohkoh spacecraft.

TRACE observes ultraviolet light from iron atoms in solar plasma at
two temperatures: 1.5 million degrees Celsius (2.7 million
Fahrenheit) and 10 million degrees Celsius (18 million Fahrenheit).
Theories explaining the tadpoles included the possibilities that they
were dense blobs of plasma, with a different temperature than what
TRACE could detect, that absorbed ultraviolet light from plasma
behind them or that they were voids with hardly any
ultraviolet-emitting plasma in them.

While both circumstances would create dark regions in TRACE images,
the new data, coordinated with observations from other spacecraft,
have now convinced scientists that the tadpoles are superheated
magnetic voids in the plasma. The voids are formed when magnetic
fields that lace the solar atmosphere reconnect and snap back to the
surface following a flare and CME. According to the analysis
presented today, the tadpoles appear dark simply because there is
very little material in them.

“Imagine a hot-air balloon lifting off the ground and stretching
elastic tethers placed over its top,” said McKenzie. “The tethers are
like the solar magnetic field, and the balloon represents the CME. As
the balloon rises, the elastic tethers stretch, get pulled together,
and start to tangle underneath the balloon. If the tethers were to
behave like solar magnetic fields, instead of simply breaking, broken
tethers would reconnect to other broken tethers, forming new
connections (magnetic reconnection).”

“If this tangling and reconnection goes on long enough, pieces of
elastic tethers that are connected to the ground (the solar surface)
connect to other ‘grounded’ segments, and subsequently snap back down
to the ground. Their snapping downwards gives the tadpoles a downward
motion, as the stretched magnetic fields relax to form long rows of
arches called arcades. Pieces connected to the balloon get tied to
other ‘balloon-connected’ segments and are carried off as magnetic
fields embedded in the CME.”

Apparently, the tadpoles are reconnected magnetic tubes, seen in
cross section. The tube’s magnetic pressure temporarily keeps the
surrounding hot plasma out, forming a void. With very little or no
plasma inside the tubes, there is no ultraviolet emission there, and
they appear as dark blobs (tadpoles) when seen in cross section.
After the reconnection, the magnetic tubes shrink away from the
departing CME. As the tubes move downward, the voids gradually fill
with hot plasma from underneath and disappear.

TRACE is able to take more detailed pictures faster, allowing
scientists to better characterize the behavior of the tadpoles. “No
one knows exactly how magnetic reconnection works. The TRACE
observations give us constraints, which allow us to select from among
many competing theories,” said Deluca.

“Improved understanding of magnetic reconnection will help us better
understand when a highly-magnetized region of the Sun will suddenly
erupt as a flare or CME,” added Deluca.

Other members of the TRACE team include lead author Mr. Fenwick
Cooper, a Ph.D. student at the University of Warwick, The United
Kingdom, Prof. Valery Nakariakov, also of the University of Warwick,
and Dr. Dan Seaton of SAO.

For images and more information, refer to:
http://www.gsfc.nasa.gov/topstory/2003/0411tadpoles.html