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Bill Steigerwald
NASA Goddard Space Flight
Center                                        July 9, 2003
(Phone: 301/286-5017)
William.A.Steigerwald{at}nasa.gov

Release: 03-70

ROCKET TELESCOPE GETS CLOSEST LOOK AT THE SUN

Scientists got their closest-ever ultraviolet look at the Sun from
space, thanks to a telescope and camera launched aboard a sounding
rocket. The images revealed an unexpectedly high level of activity in
a lower layer of the Sun's atmosphere (chromosphere). The pictures
will help researchers answer one of their most burning questions
about how the Sun works: how its outer atmosphere (corona) heats up
to over one million degrees Celsius (1.8 million Fahrenheit), 100
times hotter than the chromosphere. 

A team of Naval Research Laboratory (NRL) scientists used the Very
high Angular resolution ULtraviolet Telescope (VAULT) to take
pictures of ultraviolet (UV) light (1216 =C5) emitted from the upper
chromosphere. Resolving areas as small as 240 kilometers (150 miles
or 0.3 arcseconds) on each side, the June 14, 2002, flight captured
images about three times better than the previous-best pictures from
space. A few ground-based telescopes can observe the Sun in
150-kilometer (93-mile) increments, but only at visible wavelengths
of light. UV and X-ray wavelength observations most directly matter
to solar weather. 

Since most solar weather originates as explosions of the electrified
gas (plasma) in the corona, understanding the heating and magnetic
activity of the coronal plasmas will lead to better predictions of
solar weather events. Severe solar weather, like solar flares and
coronal mass ejections, can disrupt satellites and power grids,
affecting life on Earth. 

The VAULT observations reveal a highly structured, dynamic upper
chromosphere, with structures visible for the first time thanks to
the detailed resolution. A large number of structures in the pictures
change rapidly from one image to the next, 17 seconds later.
Scientists previously thought these changes occurred over five
minutes or more. The transience of the physical processes in this
layer has significant theoretical implications, such as the fact that
proposed heating mechanisms must now also be effective over
relatively short time scales. 

Scientists found chromospheric features in the VAULT images that
match features, based on shape and spatial correlation, which they
see in Transition Region And Coronal Explorer (TRACE) satellite
images of the corona taken simultaneously. This comparison shows that
these two layers have much higher correlation than previously thought
and implies that similar physical processes likely heat each.
However, theory predicts the activity in the chromosphere should be
lower than what scientists observed in the VAULT emissions. "[There
are] more things happening below [in the upper chromosphere] than you
see in the corona," says VAULT project scientist Angelos Vourlidas
of the NRL. 

VAULT also revealed unexpected structures in quiet areas of the Sun.
The plasma and magnetic field bubble up like boiling water on the
Sun's visible surface (photosphere), and, like bubbles gathering and
forming a ring at the edge of a pot, the field builds up in rings
(network cells) in the quiet areas. VAULT captured images of smaller
features and significant activity within the network cells,
surprising scientists. 

The telescope took 21 images in the Lyman-alpha wavelength of the
electromagnetic spectrum during a six-minute-nine-second
picture-taking window on its 15-minute flight. Offering the brightest
solar emissions, the Lyman-alpha wavelength assured the best
likelihood for pictures from the rocket and allowed for shorter
exposure times and more pictures. An increase in Lyman-alpha
radiation may indicate an increase in solar radiation reaching Earth.

The VAULT payload consists of a 30-centimeter (11.8-inch) Cassegrain
telescope with a dedicated Lyman-alpha spectroheliograph focusing
images onto a charge-coupled device (CCD) camera. The CCD, also
employed in consumer digital cameras, has a photosensitivity 320
times greater than photographic film previously used. The Normal
Incidence X-ray Telescope (NIXT) from the Harvard-Smithsonian Center
for Astrophysics took the previous best-resolution pictures of the
Sun from space in September 1989, also aboard a sounding rocket.

The scientists verified the payload performance with an engineering
flight from White Sands Missile Range, N.M., May 7, 1999. The June
14, 2002, flight from White Sands was the first scientific flight of
the payload. The NRL team led a campaign combining observations from
satellites and ground-based instruments. Scientists plan a third
launch in Summer 2004. The mission was conducted through NASA's
Sounding Rocket Program. For more information and images, visit:

http://www.gsfc.nasa.gov/topstory/2003/0708vault.html

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