On July 11th, amateur astronomers from around the world trekked to remote islands in the South Pacific to witness a total solar eclipse. They watched in awe as the Moon completely covered the Sun and revealed wispy, incandescent streamers in our star's million-degree atmosphere, the corona.

These fleeting glimpses of the corona last just a few minutes and happen only every 19 months on average. But thanks to the Solar and Heliospheric Observatory, launched in December 1995, researchers can view the Sun's atmosphere any time they want, day or night, from the comfort of their computers.

Developed jointly by NASA and the European Space Agency, SOHO and its 12 experiments monitor the Sun's brilliant disk and its corona in ways impossible from ground-based telescopes. Moreover, this sentinel's longevity has yielded observations over an entire 11-year cycle of solar activity, providing unprecedented insights about how our star works.

A Tough Sell
Remarkably, the concept of using a spacecraft to monitor the entire Sun continuously was not an easy sell, notes Joseph Gurman, Project Scientist for SOHO at NASA's Goddard Space Flight Center in Greenbelt, Md. "Earlier spacecraft were designed to follow flares and active regions on the Sun," he explains, because evidence from ground-based telescopes suggested that the solar corona changed little from day to day.

But the corona proved to be anything but boring. Instead, it's actually a seething cauldron of superheated gases and intense magnetic fields that's constantly on the move. "We now realize," Gurman says, "that the corona changes on every time scale at which we've ever observed it."

One of SOHO's corona-watching cameras - the Extreme Ultraviolet Imaging Telescope (EIT) - has been critical to this epiphany. Built by specialists in France, Belgium, Germany, and the United States, EIT takes snapshots of the Sun at four ultraviolet wavelengths. Initially these views were beamed to Earth only twice per day. But over time mission managers increased this cadence dramatically, and eventually image sets were being taken every 12 minutes.

Each of EIT's four spectral windows is attuned to invisible but very energetic photons of light emitted by ionized atoms in the corona. For example, ultraviolet light at a wavelength of 195 �ngstroms comes from iron heated to an amazing 2,700,000 degrees F (1,500,000 K) - hot enough to strip eleven electrons from each atom.

Revolutionizing Solar Science
How the lower corona gets so incredibly hot has puzzled solar physicists for decades. It's hundreds of times hotter than the Sun's light-producing "surface" layer, the photosphere, which has an average temperature of about 10,000 degrees F (5,800 K).

Prior to SOHO's launch, physicists believed that countless tiny eruptions in the photosphere were driving up the corona's temperature. But EIT observations suggest that these "nanoflares" can't supply enough energy to make the corona so hot. Some other process must be involved.

Another revelation has come from watching the Sun spew enormous blobs of matter into space. These titanic eruptions, called coronal mass ejections or CMEs, propel a million tons of magnetized, superheated ions and electrons from the lower corona (where they're seen by EIT), up through its outer regions (where they're seen by another SOHO camera, the Large Angle and Spectrometric Coronagraph Experiment), and out into the solar wind.

CMEs can arise several times each day during the Sun's peak in activity, and whenever one of these fast-moving super bubbles sweeps past Earth, our planet's protective magnetosphere recoils from the shock. The sudden assault of charged particles causes auroral displays to intensify and can sometimes damage orbiting satellites.

Apart from its scientific value, Gurman credits EIT with transforming the public's perceptions of the Sun and space weather. Ever since their debut in 1996, SOHO videos showing the Sun's dynamic face and its rotation have become tremendously popular. "In earlier missions we didn't get images of the Sun's entire disk," he explains.

"There's a visceral reaction to seeing the Sun as a place, rather than just a hot ball of light."

All told, EIT has amassed more than 500,000 images during its 14.5 years of operation. Except for a four-month hiatus in 1998, when a software glitch caused the spacecraft to veer away from the Sun and nearly doomed the mission, this long observing record has been unbroken.

"It's the 24-hour coverage of the Sun's high-temperature corona that really has been revolutionary," observes Jay Pasachoff, a solar expert at Williams College in Massachusetts.

The Next Generation
With the launch of NASA's Solar Dynamics Observatory last February, the task of keeping tabs on our dynamic star has passed to a newer and more capable spacecraft.

Two SDO instruments, the Atmospheric Imaging Assembly (AIA) and the Extreme Ultraviolet Variability Experiment (EVE), are taking solar snapshots much more often and with far greater detail than was possible with SOHO. For example, AIA records the Sun at 10 different wavelengths every 10 seconds.

The changing of the solar guard began on in earnest on August 1st. That's when SOHO was commanded to start sending EIT images to Earth only twice per day, instead of every 12 minutes.

This reduced frequency frees up radio bandwidth so that LASCO can provide enhanced imaging of the outer corona (SDO does not carry a comparable camera). Even in its diminished role, however, EIT remains crucial to SOHO's success: Since it's bolted directly to the spacecraft, it serves as the guide scope for all the other instruments.

A generation ago, solar specialists doubted the value of including EIT in SOHO's payload. The experiment's principal investigator, Jean-Pierre Delaboudiniere, struggled to find the resources to construct the instrument. Today, however, its legacy is secure.

"Had it not been for the success of EIT," Gurman observes, "I doubt there'd be any extreme-ultraviolet imagers on the Solar Dynamics observatory - or even an SDO mission."