A WINDII view of the upper atmosphere

High in the Earth's atmosphere, tidal waves of intense winds wash around the planet each day. Unlike the twice-daily ocean tides on the Earth's surface, these 24-hour atmospheric tides are driven not by the moon's gravity but by the sun's heat. The winds reach enormous speeds-hundred of kilometres per hour-but in the thin atmosphere at high altitudes, they'd probably feel like no more than a light breeze.

This unique picture of Earth's gusty and turbulent upper atmosphere emerged from data collected by a $40-million Canadian-built instrument called WINDII (Wind Imaging Interferometer). Sponsored by the Canadian Space Agency and the Centre national d'études spatiales (CNES) of France, WINDII was among 10 instruments on NASA's Upper Atmosphere Research Satellite (UARS) launched from the Space Shuttle in 1991.

"The winds turned out to be vast," said Gordon Shepherd of York University's Centre for Research in Earth and Space Science (CRESS), head of the WINDII science team. "Every day, they reach over 200 kilometres per hour on either side of the equator, which surprised us. That's like hurricane-strength winds all the time."

The fact that WINDII was even able to detect these tides was surprising. There'd been predictions they might be too small to measure but "they turned out to be the biggest thing we saw," said Shepherd. "They dominated all else."

Orbiting nearly 600 kilometres above the Earth, WINDII studied various aspects of the upper atmosphere between 80 and 300 kilometres altitude. Little was known about circulation in this region and how events occurring there influence events elsewhere in the atmosphere. This type of information is important to understand and predict global weather and climate phenomena.

"The dynamics were the last thing to be measured and the most difficult," said Shepherd. "The sources and magnitudes of the upper atmosphere winds really weren't known. We wanted to know how the winds change daily, seasonally and over the 11-year solar cycle. What we learned was quite different from what we thought before."

Scheduled to operate for just 18 months, the UARS satellite lasted 14 years. The WINDII instrument, which operated for 12 years, performed flawlessly from the outset, exceeding the most optimistic expectations and producing millions of images that scientists will be studying for decades to come. "It worked just perfectly," said Shepherd, adding that he'd have been happy with two months of data. "Certainly, 18 months was all I expected. I couldn't imagine 12 years."

Preparing WINDII for space

Preparing an instrument for space is a white-knuckle exercise with many potential pitfalls between conception and the successful gathering of data. A jarring rocket launch carries the instrument into a hostile environment featuring an almost complete vacuum, extreme temperatures and high radiation levels. It must function for years, even decades, without any hands-on doctoring. The cost of failure is measured in dollars as well as in years of effort invested by scientists and engineers.

It can take as long as 15 years of ground-based work to get a system into space, said WINDII team member Charles Hersom of Spectral Applied Research Inc. "There's lots of time pressure. You gamble a lot, cross your fingers and toes and do the best engineering you can."

WINDII was a challenge because "we were proposing to do something nobody had done before with an instrument nobody had built," said Shepherd.

The team ran into two potentially serious problems. The first involved two bolts that weren't quite long enough-and would have cost $1 million to fix. The second involved filters that shift as they age-and did cost $1 million to fix.

The bolt problem related to the position of WINDII on the side of the UARS satellite. Two bolts were too short to allow the instrument to be positioned as precisely as required, which would have caused WINDII to be pointed too high. "We would have missed an awful lot of data," said Shepherd. "But it was going to cost $1-million to change the bolts."

He came up with a less expensive option-suggesting that NASA simply lower the satellite's orbital altitude. Fortunately, the other researchers who had instruments on UARS felt such a change would not negatively affect their experiments so the plan was approved.

The problem with the filters was more serious and less easily or cheaply fixed. These filters were used to isolate the precise wavelengths of light that WINDII was supposed to measure. They shift as they age, reducing the amount of the correct wavelengths they let through; to prevent this, they're "pre-aged" on the ground. It turned out, however, that the WINDII filters hadn't been properly pre-aged.

The choices were unpalatable-either take a chance that the filters would work well enough for the planned 18 months of the mission or tear the instrument apart and put in new filters. It was risky, but that's what they did-at a cost of $1-million. "If we hadn't, it would have been a disaster," said Shepherd. After several years of satellite operations, "we'd have seen nothing. As it was, we got data to the end with very little wavelength shift."

The next big nail-biting moments came during the launch and the early operations of the instrument. Shepherd was worried that the interferometer, which contained several critical pieces of glass cemented together, could break apart during launch. There was also concern that the glass might crack because of temperature changes. Therefore, it was an enormous relief when, shortly after the launch, the ground team received confirmation that WINDII was alive and well. "To see it actually operating was overwhelming," said Shepherd.

But they weren't out of the woods yet. The next step was to open a shutter that protected WINDII's delicate optical system from particles emitted by the satellite during the first few weeks of operations. This required firing a small explosive device to trigger a series of events that released the shutter. If this procedure failed, all was lost. Fortunately, the shutter popped open as planned and data immediately streamed down from space. The WINDII team could finally crack out the champagne.

Although WINDII started gathering data right away, it also went through a process of validation that took several years. This involved comparing its data with measurements taken by ground-based instruments in Canada, the United States, Europe, Australia and New Zealand, as well as another instrument on the UARS satellite. Agreement among the instruments demonstrated that WINDII's data could be relied on.

WINDII's interferometer was the first of its kind to be flown in space so the team also documented the instrument's behaviour, watching for changes that occurred over time. This information will be valuable in designing similar instruments in future.

Throughout the years of its mission, WINDII received rave reviews for its reliability. "There were a number of mechanical parts and none of them ever failed," said Hersom. "WINDII was an exceptionally good instrument."

Measuring winds by detecting light

Because the UARS satellite orbited several hundred kilometres above where the winds occur, WINDII used remote sensing techniques to measure them. It measured airglow, a form of light in the visible spectrum, emitted by oxygen atoms and molecules in the atmosphere. It took measurements in two directions so the scientists could calculate winds moving north-south, and east-west.

WINDII detected the winds' 24-hour, or diurnal, tidal pattern caused by solar heating of the atmosphere. At low latitudes, the tide originates in the troposphere, the region nearest the Earth's surface, where the sun's energy is absorbed mainly by water vapour. The heating is strongest at noon and it "rotates westward around the Earth with a period of 24 hours, just like the sun," said WINDII team member Charles McLandress, a University of Toronto atmospheric scientist. The atmosphere responds to this "diurnal blob" of heating by generating a wave-the diurnal tide-that moves both westward and upward.

These atmospheric waves move vertically and horizontally "like waves on a sloping beach," Shepherd said. They get larger as the atmospheric density decreases with altitude; what starts off as a small ripple in the lower atmosphere "grows many, many times," said McLandress. "This blob of heating going around the Earth once a day has a relatively small impact on local temperatures and wind speeds but the effect increases as you go up." The waves peak at about 90 to 95 kilometres altitude before they dissipate.

Because the atmosphere is so thin at high altitudes, the winds wouldn't feel very strong at more than 200 kilometres an hour. "They don't pack the same force because of the low density," said McLandress. "If you were to stand up there, I don't think you would be blown over. On the ground, the air mass pushes you."

Scientists were surprised to find that the 24-hour atmospheric waves varied by season; they're strongest in spring and fall and weakest in summer and winter. The WINDII data don't explain why but McLandress was able to explain the mechanism behind the seasonal variation by comparing the WINDII data with simulations from a computer model.

WINDII also measured "planetary waves" that circle the Earth in two or more days, caused not by atmospheric heating but by the Earth's rotation. The two-day wave is particularly striking, said William Ward, a physics professor at the University of New Brunswick. It's associated with high winds and is strongest in January and July/August.

Ground-based observations indicated the existence of these waves, but "no satellite had a global view prior to WINDII," said Ward, who described them as the "sinews" of the atmosphere. "We had clues these things were up there, but nobody had a clear idea of the structure of these waves."

These waves play an important role in the circulation of the upper atmosphere and may help scientists better understand processes related to climate change and damage to the ozone layer. For example, they play a role in breaking down polar vortices over the Northern pole in winter; these whirlpools of extremely cold air are associated with the development of large ozone holes over both the North and South poles, but they're more stable in the Southern Hemisphere, creating larger and more long-lasting ozone holes there.

"The WINDII findings also explain another curious phenomenon," said Shepherd. "Nobody could understand why airglow was so variable. But we could see why; the winds were pushing atomic oxygen around, and we saw the beautiful global patterns in the airglow. The winds were turning the atmosphere over, like a boiling kettle."

Improving computer models

Scientists once thought the atmosphere was fairly static but now they know its composition is constantly changing because of the winds, Shepherd said. These data have enabled scientists to improve computer models of the upper atmosphere. "Now the motions are fully integrated into our science. They had to crank up the models to produce winds of the kind we saw."

WINDII also improved understanding of the chemistry of the upper atmosphere by providing indirect evidence of the amount of atomic oxygen in the mesosphere and its distribution by height. The mesosphere extends from about 50 to 80 or 85 kilometres in altitude. According to Robert Lowe, a University of Western Ontario physics professor, atomic oxygen doesn't last long lower in the atmosphere, but above about 85 kilometres, it lasts for a least a day. This, he said, creates "a vast store of energy. It's essentially stored sunlight that can be used up in various chemical processes. It controls most of the chemistry of that region."

He said these data will be extremely valuable in validating computer models of the behaviour of the atmosphere, and that WINDII data is helping in the effort to understand the connections and couplings between different regions of the atmosphere, which is important to understanding the Earth's climate.

In the past, scientists found it difficult to accept that events occurring so high in the atmosphere had an important influence on the weather and climate of lower regions, but this view is changing, Lowe said. "The more you study it, the more you find there are links between the regions."

McLandress added that having a good global record of current conditions in the upper atmosphere is important in order to detect and understand future changes in the Earth's climate. "With climate change, you have to characterize the present state of the atmosphere. This requires global measurements over at least a period of several years."

The experience gained during the WINDII program will be put to good use in the future. A Canadian scientific team, led by Ian McDade of York University, is working to develop another instrument called SWIFT (Stratospheric-Wind Interferometer for Transport) that will measure winds and ozone concentrations in the stratosphere, which extends from 8 to15 kilometres altitude up to 50 kilometres. The plan is to launch it in 2010.

In the latter part of its life, WINDII spent much of its time asleep because the UARS satellite experienced failures that limited the available power. Nevertheless, the instrument continued to generate data until it was shut down in September 2003. The satellite itself ceased operations in December 2005. "It will burn up in the atmosphere in a couple of years," said Shepherd.

A fitting end for an instrument whose purpose was to enhance the state of knowledge about how the atmosphere works.

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