Solar Storm’s Anatomy Probed in 3-D
Images from telescopes onboard STEREO spacecraft showing a coronal mass ejection event on December 12-13, 2008. Data from both spacecraft are shown simultaneously.
Credit: NASA

NASA's twin STEREO spacecraft have provided scientists with their first view of the speed, trajectory and three-dimensional shape of powerful explosions from the surface of the sun.

This knowledge could help scientists better understand the inner workings of these solar tsunamis, known as coronal mass ejections (CMEs), and how and when they could affect Earth.

"We are able to obtain the three-dimensional structure of the CME for the first time," said Angelos Vourlidas, project scientist for the Sun Earth Connection Coronal and Heliospheric Investigation, of the Naval Research Laboratory in Washington, D.C.

One familiar phenomenon that results when a CME collides with Earth's magnetosphere are the beautifully colored auroras, more commonly called Northern or Southern Lights. While these light shows in the sky are harmless, CMEs can produce a form of solar cosmic rays that can be hazardous to spacecraft, astronauts in space and technology on Earth.

Such space weather produces disturbances in electromagnetic fields on Earth that can induce extreme currents in wires, disrupting power lines and causing wide-spread blackouts. These sun storms can interfere with communications between ground controllers and satellites and with airplane pilots flying near Earth's poles. Radio noise from the storm also can disrupt cell phone service.

"If they hit Earth it can actually cause all sorts of electrical consequences," said STEREO project scientist Michael Kaiser of NASA?s Goddard Space Flight Center in Greenbelt, Md.

Coronal mass ejections carry billions of tons of plasma into space at thousands of miles per hour. This plasma, which carries with it some of the magnetic field from the corona, can create a large, moving disturbance in space that produces a shock wave. The wave can accelerate some of the surrounding particles to high energies that can produce a form of solar cosmic rays.

This process also create the disruptive space weather that can happen during and following the CME's interaction with Earth's magnetosphere and upper atmosphere.

The sun in stereo

NASA's Solar Terrestrial Relations Observatory, or STEREO, spacecraft have a unique view of CMEs because are stationed at different vantage points. One leads Earth in its orbit around the sun, while the other trails the planet.

The spacecraft can make simultaneous observations of these ejections of plasma and magnetic energy that originate from the sun's outer atmosphere, or corona, which enables researchers to triangulate the trajectory of the CMEs.

The unique positioning of the spacecraft combined with their monitoring abilities "really gives us insight here that we have not had before," said Antoinette Galvin, principal investigator, Plasma and Suprathermal Ion Composition instrument, from the University of New Hampshire in Durham, likening their measurements to those made of more decidedly Earth-bound storms: "They're like those aircraft that go into the eye of the hurricane."

Using three-dimensional observations, solar physicists can examine a CME's structure, velocity, mass, and direction in the corona while tracking it through interplanetary space. These measurements can help determine when a CME will reach Earth and predict how much energy it will deliver to our magnetosphere, which is Earth's protective magnetic shield.

"Before this unique mission, measurements and the subsequent data of a CME observed near the sun had to wait until the ejections arrived at Earth three to seven days later," Vourlidas said. "Now we can see a CME from the time it leaves the solar surface until it reaches Earth, and we can reconstruct the event in 3-D directly from the images."

STEREO has taken such measurements for 40 CMEs, "and we can do it for pretty much any CME that the sun is going to throw at us," Vourlidas said.

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