Astronomy Picture of the Day 07-20-04

Explanation: On June 8, Venus was not the only celestial object to pass in front of the Sun. A few well-situated photographers caught the International Space Station also crossing the Sun simultaneously. Pictured above is a unique time-lapse image of the unprecedented double transit, a rare event that was visible for less than a second from a narrow band on Earth. The above image is a combination of 12 frames taken 0.033 seconds apart and each themselves lasting only 1/10,000 th of a second. The image was taken from the small village of Stupava in Slovakia. The next time Venus will appear to cross the Sun from Earth will be in 2012.

Today is also my brother's 39th birthday. Happy birthday, Greycamel!

Satellite sees 'sprites' in the upper atmosphere
UNIVERSITY OF CALIFORNIA-BERKELEY NEWS RELEASE
Posted: July 18, 2004

Photos of red sprites, blue jets, elves and sprite halos are now flowing into the University of California, Berkeley's Space Sciences Laboratory from the first satellite instrument devoted to the study of these puzzling high-altitude lightning flashes.

A colorized photo of sprites in the upper atmosphere above a bright flash of lightning over Africa at an altitude of about 80-90 kilometers (about 55 miles), just below the airglow layer visible across the top of the image. Credit: Stephen Mende/UC Berkeley

All these phenomena are caused by the discharge of lightning from storm clouds into the upper atmosphere and ionosphere, which begins at an altitude of about 100 kilometers (63 miles). These discharges produce much different effects from the craggy lightning discharges to the ground. But little is known about them because they occur between 50 and 100 kilometers above the earth's surface, too high for airplanes to study and too low for most satellites.

The Taiwanese government, however, funded an instrument called The Imager of Sprites and Upper Atmospheric Lightning (ISUAL), which was built by a collaborative team of Taiwanese, Japanese and UC Berkeley scientists and launched aboard the Taiwanese satellite ROCSAT-2 (Republic of China Satellite 2) on May 20 of this year.

"With ISUAL, we are trying to figure out the properties of the global electrical circuit, how the lower and upper atmosphere are coupled electrically," said Stephen Mende, a physicist at the Space Sciences Laboratory and UC Berkeley's principal investigator for the project. "The first goal, however, is to find the global distribution of sprites and jets, and how often they occur."

Though the discharges are not known to have any negative effect on high-flying planes, they could have an impact on atmospheric chemistry, Mende said. Electrical discharges in the atmosphere produce nitrogen oxides (NOx), one of the agents known to degrade the protective ozone layer. If low-intensity, cloud-ionosphere discharges were occurring all the time - some 1,000 electrically active thunderstorms are going on around the world at any given time - they may be producing a significant amount of nitrogen oxides.

The first image from ISUAL came in July 4, showing red sprites - short fluorescent "tubes" glowing like a neon light - terminating in the ionosphere. Another photo that day showed a brilliant lightning flash with a trio of sprites hovering above and a sprite halo encircling it. Subsequent photos show a lightning flash inside a high-altitude storm cloud accompanied by a sprite halo sitting above it in the airglow layer of the ionosphere.

Though first reported by eyewitnesses as early as 1895, these atmospheric discharges weren't taken seriously by scientists until 1990, when video images of sprites were first published. Scientists think these discharges take place after a lightning strike to the Earth's surface, which grounds the storm cloud tops and creates a large voltage difference between the cloud and the upper atmosphere. Because the air at this altitude is thinner, the strong electrical fields can break down the air molecules much like the ionization of neon gas inside a neon light.

Since this breakdown is easiest where the air is thinnest, it typically occurs high above and detached from the cloud, often touching the ionosphere. These breakdowns are called red sprites because nitrogen, the main constituent of air, glows red when excited by low energy electrons in this type of discharge. As the sprite reaches the rarified ionosphere, it often produces a sprite halo, a diffuse glow in the layer of ionosphere above the sprite.

Jets, on the other hand, are blue and emanate from the top of a thundercloud, seemingly ejected from the core. And elves are rapid bursts of emissions of light due to electromagnetic pulses from lightning jolts. Elves appear to be similar to halos, but occur at higher altitudes, typically above 95 kilometers (60 miles), at the bottom of the ionosphere. Sprites, elves, jets or sprite halos are collectively termed "transient luminous events."

A cloud-to-ground lightning flash always precedes the sprites and jets. Mende a few years ago found a 3-10 millisecond delay from lightning flash to sprite, though last year another team found some sprites delayed as much as 100 msec. The ISUAL instruments should help put that argument to rest.

The ISUAL instruments include a spectrophotometer that measures the intensity of the light given off in six colors simultaneously. This measurement helps in determining the energy of the electrons in the current flashing between the clouds and the ionosphere and in estimating the energy and strength of the electrical field.

The ISUAL camera looking out on the starboard side of the satellite is capable of taking six photos within 6 milliseconds, each exposure lasting 1 millisecond. The six photos in quick succession allow the team to analyze the time development of the transient luminous events.

During its expected five years of operation, the satellite, which is in a nearly polar orbit at 891 kilometers (561 miles), also will spend time looking at the nighttime auroras over the north and south poles. ROCSAT-2, launched from California's Vandenberg Air Force Base aboard an Orbital Sciences Corp. enhanced-version Taurus XL rocket, also carries a French-built camera that looks downward with 2-meter resolution to study the ocean and land around Taiwan.

Mende's colleagues in building the imaging camera and the spectrophotometers were Harald U. Frey, Stewart Harris and Henry Heetderks of UC Berkeley's Space Sciences Laboratory. Another instrument, the array photometer, was built by Y. Takahashi and H. Fukunishi of Tohoku University in Japan. The principal investigator for the $5 million ISUAL instrument is Rue Ron Hsu of National Cheng Kung University in Taiwan, who worked on it with colleagues H. T. Su and Alfred Chen. The approximately $75 million satellite was funded by National Space Program Office of Taiwan.

Cassini shows the dark side of Saturn's moon Dione
CASSINI PHOTO RELEASE
Posted: July 19, 2004

The icy, cratered surface of Saturn's moon Dione shows more than just its sunlit side in these two processed versions of the same image.

Credit: NASA/JPL/Space Science Institute
Download a larger image version here

The view at left, with only mild enhancement, shows a romantic crescent with large craters visible. The contrast in the version at the right has been greatly enhanced to show the side of Dione lit faintly by reflected light from Saturn. A similar phenomenon can be seen from Earth, when the Moon's dark side is visible due to "earthshine." The crater at the top of the image appears to have a sunlit central peak in the enhanced view -- a common characteristic of craters on Dione as seen in Voyager images. Slight variations in brightness on the moon's dark side hint at the bright curved linear streaks, seen by Voyager. These streaks are thought to be deposits of water ice.

The image was taken with the Cassini spacecraft narrow angle camera on July 2, 2004, from a distance of about 1.4 million kilometers (860,000 thousand miles) from Dione, at a Sun-Dione-spacecraft, or phase angle of about 119 degrees. The image scale is 8 kilometers (5 miles) per pixel. Dione's diameter is 1,118 kilometers (695 miles) across. The images have been magnified by a factor of two to aid visibility.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The Cassini orbiter and its two onboard cameras, were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

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