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 Angstroms) 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:
