Spacecraft sees solar storm engulf Earth

For the first time, a spacecraft far from Earth has turned and watched a solar storm engulf our planet. The movie, released today during a NASA press conference, has galvanized solar physicists, who say it could lead to important advances in space weather forecasting. The movie shows a CME swelling into an enormous wall of plasma and then washing over the tiny blue speck of Earth where we live.

CMEs are billion-ton clouds of solar plasma launched by the same explosions that spark solar flares.   When they sweep past our planet, they can cause auroras, radiation storms, and in extreme cases power outages.  Tracking these clouds and predicting their arrival is an important part of space weather forecasting.

STEREO-A is one of two spacecraft launched in 2006 to observe solar activity from widely-spaced locations. At the time of the storm, STEREO-A was more than 65 million miles from Earth, giving it the “big picture” view other spacecraft in Earth orbit have been missing.

When CMEs first leave the sun, they are bright and easy to see. Visibility is quickly reduced, however, as the clouds expand into the void.  By the time a typical CME crosses the orbit of Venus, it is a billion times fainter than the surface of the full Moon, and more than a thousand times fainter than the Milky Way.  CMEs that reach Earth are almost as gossamer as vacuum itself and correspondingly transparent.

Footage of the stor was recorded back in December 2008, and the team have been working on it ever since.  Now that the technique has been perfected, it can be applied on a regular basis without such a long delay.

“Until quite recently, spacecraft could see CMEs only when they were still quite close to the sun. By calculating a CME’s speed during this brief period, we were able to estimate when it would reach Earth. After the first few hours, however, the CME would leave this field of view and after that we were ‘in the dark’ about its progress.”Alysha Reinard of NOAA’s Space Weather Prediction Center explains the benefits for space weather forecasting

The ability to track a cloud continuously from the Sun to Earth is a big improvement. In the past, our very best predictions of CME arrival times had uncertainties of plus or minus 4 hours. The kind of movies we’ve seen could significantly reduce the error bars.

The movies pinpoint not only the arrival time of the CME, but also its mass.  From the brightness of the cloud, researchers can calculate the gas density with impressive precision.  Their results for the Dec. 2008 event agreed with actual in situ measurements at the few percent level.  When this technique is applied to future storms, forecasters will be able to estimate its impact with greater confidence.

The kind of magnetic transformations revealed by the movie deeply impressed Guhathakurta:  “I have always thought that in heliophysics understanding the magnetic field is equivalent to the ‘dark energy’ problem of astrophysics.  Often, we cannot see the magnetic field, yet it orchestrates almost everything.   These images from STEREO give us a real sense of what the underlying magnetic field is doing.”

The inner physics of CMEs have been laid bare for the first time—a development that will profoundly shape theoretical models and computer-generated forecasts of CMEs for many years to come.  (Science NASA)