March 6, 2008: Imagine living on a planet where Northern Lights fill the heavens at all hours of the day. Around the clock, even in broad daylight, luminous curtains shimmer and ripple across the sky, mesmerizing anyone who bothers to look.

News flash: Astronomers have discovered such a planet. Its name is Earth.

"Our own planet has auroras 24 hours a day," says Jim Spann of the Marshall Space Flight Center, "and we can see them even in broad daylight." The trick, he explains, is picking the right wavelength. "If we look at Earth from space using an ultraviolet (UV) filter, we see there are auroras underway at all times. It is a beautiful sight."



The extent of the phenomenon was revealed twelve years ago by NASA's Polar satellite. As its name suggests, Polar orbits over Earth's poles, slowly arcing as high as 57,000 km above the arctic where it can take a good long look at the auroras below. Polar is equipped with special UV filters that allow it to photograph auroras through the glare of sunlight and "we were amazed to see Northern and Southern Lights active non-stop," says Spann, one of the scientists who led development of the satellite's UV instrumentation.

As an example he offers this movie of a recent auroral display over Antarctica. Just before the video, a solar wind gust buffeted Earth's magnetic field causing a mild geomagnetic storm. Visible light auroras—the kind we see with the human eye--were weak, but "Polar's UV camera picked up a magnificent outburst," says Spann.



Watch the movie again; it highlights an ancient mystery. Long ago, arctic sky watchers noticed that mild auroras—the ho-hum kind they saw almost every night--would sometimes erupt without warning in a stunning riot of light and color. 20th-century scientists called these events, with some understatement, "substorms." A good substorm can unleash a hundred thousand billion (1014) Joules of energy, as much as a magnitude 5 earthquake. Although auroras, generally speaking, are understood (they are caused by solar activity), the sudden power of substorms is one of the biggest mysteries of space science.

The Antarctic outburst is a classic substorm. "We see plenty of them at UV wavelengths," says Spann. "Polar's ability to monitor both night and day allows us to catch substorms that other satellites might miss."

This ability is being put to good use. Polar is now assisting THEMIS, a fleet of five spacecraft launched by NASA in Feb. 2007 to solve the mystery of the substorm. What triggers the events? Where does their power come from? These are some of the questions THEMIS has set out to answer.

The five THEMIS satellites are equipped with sensors to map the complex ebb and flow of particles and fields in Earth's magnetosphere. (The magnetosphere is a vast magnetic bubble around Earth. It is the "force field" that protects us from solar wind, and which lights up with auroras when solar wind gusts come crashing into it.) While a single spacecraft might be confused by the magnetosphere's suddenly shifting electrodynamics, the THEMIS quintet, working together, is able make sense out of very complicated events. Researchers hope this will lead to an understanding of the substorm phenomenon.

Polar is valuable because no other spacecraft can match its global view of the auroras. "We provide the big picture while THEMIS sorts out the crucial details."

At an age of 12 years, Polar is well past its design lifetime. "It's amazing that we're still in business," notes Spann. Moreover, by joining THEMIS, the veteran spacecraft is on the verge of new discovery.

Even for Polar, however, the auroras must stop sometime. During the writing of this story, the spacecraft ran out of fuel, limiting its ability to track Earth's poles. Nevertheless, mission planners believe they can squeeze another one or two month's worth of observing out of Polar in support of THEMIS. Its final images may be key pieces to the auroral puzzle.

Farewell, Polar--and thanks for all the substorms!




THEMIS -- home page

Polar -- home page

The Mystery of Auroras

NASA's Future: The Vision for Space Exploration

Source: Science@NASA

March 20, 2008: What are the signs of spring? They are as familiar as a blooming Daffodil, a songbird at dawn, a surprising shaft of warmth from the afternoon sun.

And, oh yes, don't forget the aurora borealis.

Spring is aurora season. For reasons not fully understood by scientists, the weeks around the vernal equinox are prone to Northern Lights. Canadians walking their dogs after dinner, Scandinavians popping out to the sauna, Alaskan Huskies on the Iditarod trail—all they have to do is look up and behold, green curtains of light dancing across the night sky. Spring has arrived!

This is a bit of a puzzle. Auroras are caused by solar activity, but the Sun doesn't know what season it is on Earth. Yet it seemed to know on March 1st when these lights erupted over Tromso, Norway:



"It was a very powerful outburst of Northern Lights," says photographer Bjorn Jorgensen. "The ground actually turned green!"

Such outbursts are called auroral substorms and they have long puzzled space physicists. Often sighted in springtime, "substorms erupt with little warning and sometimes shocking intensity," says UCLA space physicist Vassilis Angelopoulos. "They're a big mystery."

Angelopoulos is the Principal Investigator of NASA's THEMIS mission--a fleet of five spacecraft launched in Feb. 2007 to study the substorm phenomenon. NASA's Polar spacecraft, which can detect auroras in broad daylight using special UV filters, has also joined the effort.

It is a puzzle worthy of many spacecraft. Underlying each display of pretty lights is a potent geomagnetic storm. THEMIS observed one recently with a total energy of five hundred thousand billion (5 x 1014) Joules. "That's approximately equivalent to the energy of a magnitude 5 earthquake," says Angelopoulos. Possible side-effects of such storms range from satellite malfunctions to home power outages; telecommunications, air traffic and GPS systems are all vulnerable. "In a society that relies increasingly on space technology, understanding substorms is vital."

THEMIS may have found the substorm power supply--and a springtime connection:

"The satellites have detected magnetic 'ropes' connecting Earth's upper atmosphere directly to the Sun," says Dave Sibeck, project scientist for the mission at the Goddard Space Flight Center. "We believe that solar wind particles flow in along these ropes, providing energy for geomagnetic storms and auroras.".


Above: A magnetic map of a magnetospheric
"rope" observed in cross-section by the THEMIS
satellites on May 20, 2007. [Larger image]

It turns out that rope-like magnetic connections between Sun and Earth are favored in springtime. It's a matter of geometry: As Earth goes around in its orbit, Earth's tilted magnetic poles make different angles with respect to the Sun, tipping back and forth with a one-year cadence. Around the time of the equinox, Earth's magnetic field is best oriented for "connecting-up" with the Sun.

But wait, there are two equinoxes, spring and fall, with similar magnetic geometry. Indeed, studies show that fall is aurora season, too. Geomagnetic disturbances are almost twice as likely in spring and fall vs. winter and summer, according to 75 years of historical records analyzed by solar physicist David Hathaway of the Marshall Space Flight Center: diagram.

So, 'tis the season for auroras--and lots of data for THEMIS. Says Sibeck, "we welcome the spring!"




Spring is Aurora Season, the movie -- from the Goddard Space Flight Center (15 MB)

THEMIS -- home page

Polar -- home page

The Mystery of Auroras

NASA's Future: The Vision for Space Exploration

Source: Science@NASA