Time Space Science

The eerie beauty of the northern and southern lights has evoked visions of the supernatural for centuries: foxes of fire whisking their tales, the fighting souls of dead warriors or ancestors dancing around a ceremonial fire.

The English poet Sir Walter Scott in 1805 conjured up otherworldly beings when he wrote, "He knew, by the streamers that shot so bright, That spirits were riding the northern light."

But it was a French scientist, not a poet, who named the sight after Roman gods. In 1621, Pierre Cassendi paired Aurora, goddess of dawn, with Boreas, god of the north wind, to christen the northern lights “aurora borealis.” Those centered above the South Pole are called aurora australis for “southern dawn.”

Even today, scientists and forecasters at NOAA’s Space Weather Prediction Center in Boulder, Colo., look beyond the Earth itself for the first step in a chain of events that ultimately paints brilliant hues across the night sky at opposite ends of the planet.

Anatomy of an Aurora

Deep within the Sun, 93 million miles away, roiling plasma rises and bursts through the solar atmosphere, sometimes thrusting highly charged protons and electrons our way. When this so-called solar wind arrives near Earth, it energizes protons and electrons trapped in the planet’s magnetic field.

These charged particles then travel down magnetic field lines, like beads slipping along a string, into Earth’s upper atmosphere near the poles. There the particles in turn excite atoms and molecules of oxygen, nitrogen, and other atmospheric gases. As these atoms relax back down into their normal state, they release the excess energy as visible light, forming an aurora oval loosely centered on the magnetic pole.

During an aurora, vivid arcs, curls, waves and bands of green, red, and sometimes blue dance across the sky for minutes or hours, peaking near midnight — all between 60 and 600 miles above the ground.

Many people think auroras are rare events, but there’s almost always an aurora of some size in the sky near the poles. Seeing one is another matter.

Auroras are most often visible in regions bordering the Arctic Circle: Canada, Alaska, northern Greenland, the Scandinavian coast, and Siberia. In the south, you need to be visiting Antarctica to see an aurora frequently. But the larger the solar storm reaching Earth’s upper atmosphere, the farther the aurora extends from the poles. Residents of New England or southern Chile might see an aurora every few years. If you live in Florida or Italy, you’d be lucky to see an aurora once in your lifetime.

How Space Weather Affects Us

One of the nation’s critical operations centers, NOAA’s Space Weather Prediction Center keeps a close eye on solar activity that precedes an aurora. When a major storm explodes on the sun, followed by a suddenly intensified solar wind heading toward Earth, the center alerts airlines, the military, the communications industry, power companies and the media that a storm is on its way.

Why do NOAA scientists care about this odd “weather” on the sun and in space? NOAA monitors solar storms because they can disrupt satellite functions, power grid operations, GPS signals, high-frequency communications used by airlines and the military, and other space-based technologies that we depend on. Solar radiation could also threaten astronauts’ safety if they happen to be outside the space shuttle as it zooms past.

Visit the Space Weather Prediction Center’s aurora Web site to view the current shape and size of the auroras around the two poles. If the auroras shown there are exceptionally large and you’re in a far northern or far southern latitude, look for those spirits hurtling across the midnight sky!

Tips on Seeing an Aurora

Best time of night: 10:00 p.m. to 2:00 a.m.
Best conditions: clear night with no moon and far from light pollution
Best season: mid-winter
Best phase of the solar cycle: maximum
Best years in the sun’s current cycle: 2012 to 2013
Best position on Earth: far northern or southern latitudes


http://www.noaa.gov/

The "coming of age" of galaxies and black holes has been pinpointed, thanks to new data from NASA's Chandra X-ray Observatory and other telescopes. This discovery helps resolve the true nature of gigantic blobs of gas observed around very young galaxies.

About a decade ago, astronomers discovered immense reservoirs of hydrogen gas -- which they named "blobs" – while conducting surveys of young distant galaxies. The blobs are glowing brightly in optical light, but the source of immense energy required to power this glow and the nature of these objects were unclear.

A long observation from Chandra has identified the source of this energy for the first time. The X-ray data show that a significant source of power within these colossal structures is from growing supermassive black holes partially obscured by dense layers of dust and gas. The fireworks of star formation in galaxies are also seen to play an important role, thanks to Spitzer Space Telescope and ground-based observations.

"For ten years the secrets of the blobs had been buried from view, but now we've uncovered their power source," said James Geach of Durham University in the United Kingdom, who led the study. "Now we can settle some important arguments about what role they played in the original construction of galaxies and black holes."

Galaxies are believed to form when gas flows inwards under the pull of gravity and cools by emitting radiation. This process should stop when the gas is heated by radiation and outflows from galaxies and their black holes. Blobs could be a sign of this first stage, or of the second.

Based on the new data and theoretical arguments, Geach and his colleagues show that heating of gas by growing supermassive black holes and bursts of star formation, rather than cooling of gas, most likely powers the blobs. The implication is that blobs represent a stage when the galaxies and black holes are just starting to switch off their rapid growth because of these heating processes. This is a crucial stage of the evolution of galaxies and black holes - known as "feedback" - and one that astronomers have long been trying to understand.

"We're seeing signs that the galaxies and black holes inside these blobs are coming of age and are now pushing back on the infalling gas to prevent further growth," said coauthor Bret Lehmer, also of Durham. "Massive galaxies must go through a stage like this or they would form too many stars and so end up ridiculously large by the present day."

Chandra and a collection of other telescopes including Spitzer have observed 29 blobs in one large field in the sky dubbed "SSA22." These blobs, which are several hundred thousand light years across, are seen when the Universe is only about two billion years old, or roughly 15% of its current age.

In five of these blobs, the Chandra data revealed the telltale signature of growing supermassive black holes - a point-like source with luminous X-ray emission. These giant black holes are thought to reside at the centers of most galaxies today, including our own. Another three of the blobs in this field show possible evidence for such black holes. Based on further observations, including Spitzer data, the research team was able to determine that several of these galaxies are also dominated by remarkable levels of star formation.

The radiation and powerful outflows from these black holes and bursts of star formation are, according to calculations, powerful enough to light up the hydrogen gas in the blobs they inhabit. In the cases where the signatures of these black holes were not detected, the blobs are generally fainter. The authors show that black holes bright enough to power these blobs would be too dim to be detected given the length of the Chandra observations.

Besides explaining the power source of the blobs, these results help explain their future. Under the heating scenario, the gas in the blobs will not cool down to form stars but will add to the hot gas found between galaxies. SSA22 itself could evolve into a massive galaxy cluster.

"In the beginning the blobs would have fed their galaxies, but what we see now are more like leftovers," said Geach. "This means we'll have to look even further back in time to catch galaxies and black holes in the act of forming from blobs."

These results will appear in the July 10 issue of the Astrophysical Journal. NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.


http://chandra.harvard.edu/