A total solar eclipse passed over some of Earth’s most densely populated regions today (pictures of today’s total solar eclipse).

Just before daybreak the solar eclipse began over India and was widely visible across Asia before darkening southern Japan and finally ending over the Pacific Ocean.

Lasting as long as 6 minutes and 39 seconds in some areas, the solar eclipse boasted the longest period of totality—when the sun appears to be completely blotted out by the moon—of the entire 21st century.

Today’s record eclipse, though—if you include the periods where the sun was only partially covered—will be eclipsed by another total solar eclipse in 2132.

Solar Eclipse Science

At a remote hotel at an altitude of about 3,000 feet (900 meters) on Tianhuangping, a mountain outside the Chinese city of Hangzhou, solar eclipse scientist Jay Pasachoff witnessed the July 2009 solar eclipse with other scientists and enthusiasts from around the world.

“We saw it! The clouds kept getting thinner, and we even had a pretty good-sized hole in the clouds for the five minutes of totality, said Pasachoff,” chair of the International Astronomical Union’s Working Group on Solar Eclipses, in a blog entry.

The roughly five and a half minutes of total solar eclipse seen from Tianhuangping was the longest period of totality visible from the Asian mainland.

From a scientific perspective, “once you have five minutes-plus of totality, the extra minute that we could have [seen from the Pacific] is not significant,” Pasachoff, of Williams College in Massachusetts, told National Geographic News earlier this month. Those few minutes should have provided enough data to keep his team busy for months, he said.

Solar Mystery

Pasachoff studies solar eclipses in part to figure out why the sun’s corona—gas that extends millions of miles out from the sun—is millions of degrees hotter than the sun. The sun’s temperature is just about 6,000 degrees Fahrenheit (3,300 degrees Celsius).

“Somehow energy has been put up into the corona from lower down, heating the gas, and we’d like to see how that happens,” he said.

Scientists believe the coronal phenomenon has to do with the sun’s magnetic field, and Pasachoff is looking to identify vibrating magnetic waves that move from the sun out into the corona.

(Related: “‘Corkscrew’ Waves Seen on Sun — Keys to Solar Mystery?”)

Scientists can’t usually see the corona from Earth because its light is fainter than the blue sky created by our atmosphere.

Furthermore, instruments attached to space satellites can’t isolate all areas of the corona because the sun and the light it scatters are too bright.

The only time certain observations are possible is during a solar eclipse, when the moon blocks out the sun, creating a darker sky, which highlights the coronal light around the sun.

(Find out some of the ways eclipse chasers have advanced science, from proving Einstein’s theories to finding “other Earths” outside the solar system.)

Although the sun is about 400 times bigger than the moon, it’s also about 400 times more distant. So from the ground, the moon appears to be just a little bigger than the sun—exactly what happened as the clouds parted over Tianhuangping earlier today.

“Everyone saw all the coronal phenomena,” said Pasachoff, a National Geographic Society Committee for Research and Exploration grantee.

The distinct, jewel-like bursts of light just before and after the total solar eclipse were a highlight, he added. “The diamond rings were spectacular [picture of a solar eclipse diamond ring]. Just before totality, the clouds were just the right thickness that allowed us to see partial phases without filters.”

“All our equipment seems to have worked, so now we still have an hour or so of partial eclipse to image, and then we will download photos and start looking at them,” Pasachoff said by email from China, according to a statement.

(See some of Passachoff’s pictures of the July 22 total solar eclipse.)

Eclipse Chasers

The sun’s disappearing act attracts so-called eclipse tourists, who travel the world to watch solar eclipses, which happen between two and five times a year, though total solar eclipses are less frequent.

(See solar eclipse pictures.)

Rollie Anderson, a retired actuary from St. Louis, Missouri, is in China today, on a trip planned around the solar eclipse, his 14th.

“The cosmic coincidence that the sun and moon both appear in the sky as the same size, and then, on top of that, they line up every now and again. … Just the very idea of that is pretty mind-blowing,” he told National Geographic News earlier this month.

“As you get to the last several minutes before totality, that’s when your eyes actually start noticing things getting dark around you, and you can feel the air cooling,” he said. “It gets really dark and totality appears, and that’s when it gets most spectacular.”

“You see a black hole in the sky where the sun used be, and if there are birds around, they may stop chirping, because they think it’s night.”

Chasing eclipses has also allowed Anderson and his wife to see the world.

“It’s kind of an excuse to see whatever the part of the world the eclipse happens to be in.”

Thousands of miles above Earth, a cosmic chorus is filling the heavens with a mysterious, low frequency “hiss.”

That’s the conclusion of scientists studying data from a set of NASA probes designed to monitor substorms—dramatic exchanges of energy among charged particles that spark the auroras at Earth’s poles.

The charged particles come from the sun and get trapped in loops around our planet by Earth’s magnetic field.

Knowing how the hiss influences the loops, known as Van Allen radiation belts, might help scientists predict their behavior—a good thing, because the belts can bombard satellites, spacecraft, and even spacewalking astronauts with dangerous radiation.

Although we’re currently experiencing an unprecedented lull in solar activity, space is expected to get much stormier after 2012, when the sun should enter an active phase that will hurl more charged particles toward Earth.

Lucky Break

The faint “shh” sound that scientists now call the plasmaspheric hiss is the result of an electromagnetic wave in Earth’s radiation belts.

The hiss wave appears to reduce levels of dangerous electrons in the radiation belts by deflecting the particles from their stable trajectories and sending them into the dense upper atmosphere, where they are lost.

For more than four decades scientists have been puzzled by what was generating the hiss wave, noted study leader Jacob Bortnik, of the University of California, Los Angeles.

A previous model had suggested that the hiss wave might evolve from a more distant radio wave called chorus, so named because its discoverers in the 1950s thought it sounded like “a rookery of birds heard from a distance,” Bortnik said.

But proving this idea presented a challenge. Researchers would need simultaneous, high-resolution observations from two sophisticated satellites recording both hiss and chorus from different locations at a moment when Earth’s magnetic field was particularly active.

By chance, one of Bortnik’s students found exactly what they needed in data from two of NASA’s five THEMIS satellites.

“We didn’t think that we’d be lucky enough to get this kind of gift from nature,” Bortnik said.

The probes showed a definite correlation between the two waves, confirming that the hiss wave comes from chorus.

Weather Forecasts

Understanding the hiss wave’s origins could allow scientists to build more accurate models of the radiation belts, which could ultimately help predict space weather.

Studying space weather, Bortnik added, is similar to studying weather patterns here on Earth.

“Step number one is just to understand the system,” he said, and then maybe there’s hope that researchers can predict things in time to avoid danger.

Findings published this week in the journal Science.