While most eyes turn skyward to watch Thursday's solar eclipse (with the appropriate filters to protect your eyes) one satellite will look earthward to watch the moon's shadow race across the globe.
"It's a unique perspective," said Dr. James Spann, a co-investigator for the Ultraviolet Imager (UVI) aboard the Polar spacecraft.
UVI images will be posted here as they are received. In addition, an animated image showing the last two hours of images will be posted so visitors can see a time-lapse movie of the eclipse shadow.
Geostationary weather satellites have taken striking images of many eclipse shadows on the Earth since the 1960s, the pictures have limited scientific value, however. In the case of Polar, the spacecraft's cameras may be able to examine some of the chemistry of the upper atmosphere as the air molecules are suddenly put in the dark then re-exposed to light.
"Right now we're not sure about the science" because the method is new, said Spann at NASA's Marshall Space Flight Center. This will be the first eclipse UVI has observed.
UVI scientists hope to examine changes in the upper atmosphere by modeling the changes in airglow seen during the eclipse. Airglow is just that, an effect caused by solar ultraviolet light striking the atmosphere. By observing how the glow is extinguished then ignited as the Moon's shadow moves across the globe, Spann hopes that he and other scientists will be able to estimate oxygen densities at different altitudes in the upper atmosphere.
Polar, launched in 1996, was designed to study how the Earth interacts with the space environment. As its name indicates, most of the spacecraft's energies are focused on the Polar Regions where the magnetic field lines leave the Earth open to space. (Marshall's other instrument, the Thermal Ion Dynamics Experiment, studies ionized gases flowing upward from Earth into space. It will not be involved in eclipse observations.)
Three cameras - Marshall's UVI, the University of Iowa's Visible Imaging System (VIS), and Lockheed Martin's X-ray imager called PIXIE - are designed to observe the aurora borealis as the polar spacecraft arcs over the North Pole.
Fortunately, the satellite's position will be just right so that with a little change in attitude, the cameras can observe the Moon's shadow during the eclipse. (Looking at the Sun during the eclipse is out of the question; it would fry their image intensifiers.)
In a sense, an eclipse is always under way since the Moon always casts a shadow in space. A few times a decade, the motions of the Earth and Moon position them so the shadow is cast across the Earth.
The eclipse will take place from 1719 to 1909 Universal Time (12:19 to 2:09 p.m., Eastern time) as the shadow races from sunrise in the central Pacific, then across north end of South America, the Caribbean Sea, and the Atlantic Ocean, ending at sunset off the northwest coast of Africa. Observers on the centerline of the eclipse path will be treated to nearly 4 minutes of totality.
The animation at right, built from images courtesy of Dr. Lou Frank, VIS principal investigator at the University of Iowa, depicts Polar's view of the Earth and the track of the shadow, from 1719 to 1900 UT, as the spacecraft rises towards the North Pole. The spot size indicates location and does not necessarily match the size of the shadow.
During this period, the Moon's shadow will be in the UVI's field of view. The Polar spacecraft camera platform will be positioned to begin viewing at 1544 UT (at 10:44 a.m., before the eclipse).
Initially, the UVI will observe with a filter that admits light at 130.4 nm wavelength (visible light spans from 700 nm [red] to 300 nm [violet]). This first filter is used to adjust settings so the electronics can compensate for a slight wobble in the spacecraft.
From 1838 UT to 1851 UT, the UVI will cycle through each of its 4 filters: 130.4 nm, 135.6 nm, 140 to 150 nm, and 160 to 180 nm. The latter two are broad bands called LBH after the initials of the discoverers. The camera will spend 36 seconds taking two pictures through each filter, then step to the next one, and repeat the sequence. From 1851 UT to 1930 UT, UVI will observe with just the 130.4 nm ultraviolet filter, then return to normal observations of the aurora with the two LBH filters.