3-D solar eruptions

When they’re blasted toward Earth, solar eruptions known as coronal mass ejections (CMEs) can disrupt radio communications, satellite links, and power systems. To better gauge the angle at which CMEs emerge from the sun and predict their arrival time at Earth, NASA plans to launch in 2006 a pair of spacecraft called STEREO (Solar-Terrestrial Relations Observatory). By examining CMEs from two angles, the twin craft will record the three-dimensional structure of these eruptions. But two researchers, using data from a single craft already in orbit, have already accomplished this feat.

In the July 2 Science, Joseph M. Davila, and his colleague Thomas G. Moran of NASA’s Goddard Space Flight Center in Greenbelt, Md., describe their method, which compares several two-dimensional images of an eruption recorded by the Solar and Heliospheric Observatory through a polarizing filter (SN: 7/31/04, p. 74: Available to subscribers at Parting Shots).

Although light from the sun isn’t polarized, it becomes so when photons bounce off electrons in various parts of a CME. CME structures polarize more light the greater their angle from the line of sight between Earth and the sun. Measuring the ratio of polarized to unpolarized light for each part of a CME therefore provides the missing third dimension by giving the distance of that component from Earth.

With the technique, the team has confirmed that CMEs directed toward Earth take the shape of an expanding series of loops.