A Surprising CME

A collapsing solar filament crashed into the surface of the Sun yesterday, spawning a coronal mass ejection that could trigger aurora here on Earth.

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Sept. 13, 2000 -- On Monday, Sept. 11, the sunspot number dropped to its lowest value of the year. The face of the Sun looked remarkably blank with only a few tiny spots visible in telescopes. It was a remarkable sight considering that the Sun is well into the maximum phase of its 11-year sunspot cycle. The Sun has been almost constantly peppered with spots since "Solar Max" began earlier this year.

Yesterday, just when it seemed that a brief interlude in solar activity was at hand, the Sun unleashed a brilliant full-halo coronal mass ejection (CME).

Right: Coronagraphs on board the orbiting ESA/NASA Solar and Heliospheric Observatory recorded a full-halo coronal mass ejection at 1230 UT on Sept. 12. In this image, the faint red-colored circle shows the true size of the Sun. The solid brick-colored region denotes an occulting disk that blocks the bright glare of the Sun to reveal the fainter corona. [600 kb gif animation]

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"This CME appeared after a solar filament collapsed and dropped relatively cool, dense gas onto the surface of the Sun," says David Hathaway, a solar physicist at the NASA Marshall Space Flight Center. "You don't need sunspots to have filaments, so these types of explosions can occur even when the sunspot number is low."

The CME left the Sun traveling at about 1000 km/s, says Simon Plunkett, an operations scientist with the Solar and Heliospheric Observatory (SOHO) coronagraph team at the Naval Research Laboratory and the Goddard Space Flight Center. "At this speed it could arrive sometime late Wednesday or early Thursday."

Coronal mass ejections can carry up to 10 billion tons of electrified gas. "Halo events" like this one are CMEs aimed in the general direction of Earth. As they loom larger and larger they appear to envelop the Sun, forming a halo around our star.

Although CMEs may sound menacing, they pose little danger to Earthlings. Indeed, many skywatchers greet news of a full-halo event with enthusiasm because CMEs can trigger memorable displays of aurora when they strike Earth's magnetosphere (a region of space around Earth protected by our planet's magnetic field). The last time this happened was August 12, 2000, when the shock wave from a CME arrived at Earth just hours before the peak of the annual Perseid meteor shower. Legions of meteor-watchers saw a brilliant auroral display in spite of glaring light from that night's nearly-full Moon.

Left: Wade Clark captured this image of the Big Dipper shining through green-colored auroral sheets in the skies above Skagit County, Washington State on August 12, 200. The moonlit peak is Mt. Baker.

The situation this week could be much the same. Many stargazers will be out to see the late-summer Harvest Moon, so the CME could arrive on a moonlit night while many people are looking at the sky.

"The incoming CME might produce an auroral display, but there's no guarantee," cautioned Gary Heckman of the NOAA Space Environment Center. "There was an event very similar to this one on July 29, 1973, during Skylab. It was a very large flare, but it produced only a minor geomagnetic disturbance. It's hard to make a definitive forecast because this type of eruption is so rare."

"Another problem [for aurora enthusiasts] is that bright moonlight will render faint aurora practically invisible," added Heckman. "Still, it's worth looking just in case."

Above Left: This image of the Sun captured at the Big Bear Solar Observatory through a red Hydrogen-alpha filter shows many dark linear filaments on Sept 11, 2000. One of them near the central meridian is missing in the Sept 12th image (Above Right). During the interval between the two exposures, the filament collapsed, spawning a powerful x-ray solar flare and a brilliant full-halo coronal mass ejection.

The filament that collapsed yesterday was one of many on the Sun that space weather forecasters have been monitoring. With very few sunspots in evidence this week, collapsing filaments are the most likely source of solar activity.

"Filaments are formed in magnetic loops that hold relatively cool, dense gas suspended above the surface of the Sun," explains Hathaway. "When you look down on top of them they appear dark because the gas inside is cool compared to the hot photosphere below. But when we see a filament in profile against the dark sky it looks like a giant glowing loop -- these are called prominences and they can be spectacular."

Right: Last year extreme ultraviolet cameras on board the orbiting Solar and Heliospheric Observatory captured this image of a prominence above the eastern limb of the Sun. [more information]

"Filaments collapse when the magnetic field in their vicinity becomes unstable," explained Hathaway. "This could happen, for example, if new magnetic field lines begin to poke through the Sun's surface beneath the filament." The resulting explosions, which often occur well away from spotted regions, are called Hyder flares, named for Charles Hyder, who published studies of such events in 1967.

NASA astronauts are currently in orbit on a mission to prepare the International Space Station (ISS) for habitation. Are they in any danger from the approaching CME?

"They're in no danger," says Hathaway. "The Space Shuttle and the ISS are in low Earth orbit, well inside the magnetosphere. The astronauts are protected by the magnetosphere and to some extent by their own spacecraft. In fact, they're in a great position to observe aurora if a geomagnetic storm breaks out."

For more information about solar activity and updates about the approaching CME, please visit the NOAA Space Environment Center or Spaceweather.com.

NOAA Space Environment Center -official forecaster of space weather events

SpaceWeather.com -daily updates and news about solar flares, coronal mass ejections and geomagnetic activity

More about the "magnetotail" and what causes aurora - from the NASA/Goddard Space Flight center

All about aurora - from the University of Alaska Geophysical Institute

Aurora FAQ - from the University of Alaska Geophysical Institute

Thursday's Classroom -- lesson plans and educational activities about space weather. - from NASA/MSFC