Using data from the ESA/NASA SOHO observatory, scientists have produced the first three-dimensional (3D) views of massive solar eruptions, called Coronal Mass Ejections (CMEs). When directed at Earth, CMEs can disrupt radio communications, satellite links and power systems. This new result is critical for a complete understanding of these dramatic phenomena.
CME as seen by LASCO
Figure 1. A classical three-part CME inside the LASCO C3 field of view, showing a bright frontal loop (shaped like a lightbulb)surrounding a dark cavity with a bright core. This CME is headed roughly 90 degrees away from Earth. The uniform disk in the centre of the image is where the occulter is placed, blocking out all direct sunlight. The approximate size of the Sun is indicated by the white circle in the middle. Click here
CME as seen by LASCO
Figure 2. A similar CME heading almost directly towards Earth, observed by LASCO C2 which has a smaller field of view than C3. The size of the Sun is indicated by the larger circle, and the x-marked circle on the Sun shows the origin of the CME. Panel a shows the total intensity (darker means more intensity) as imaged directly by LASCO. Only the narrow lower end of the 'lightbulb' shape is visible - the widest portion has expanded beyond the field of view, whereas the front part and the core are too dim to be seen or hidden behind the occulter. Panel d is a topographic map of the material shown in panel a. The distance from the plane of the Sun to the material is colour coded - the scale in units of solar radii is shown on the side. Panels b and c show the intensity as it would have appeared to an observer positioned to the side of the Sun or directly above it, respectively. Click here
CMEs are the most powerful eruptions in the Solar System, with thousands of millions of tonnes of electrified gas being blasted from the Sun's atmosphere into space at millions of kilometres per hour. Researchers believe that CMEs are launched when solar magnetic fields become strained and suddenly 'snap' to a new configuration, like a rubber band that has been twisted to the breaking point.
To fully understand the origin of these powerful blasts and the process that launches them from the Sun, scientists need to see the structure of CMEs in three dimensions. "Views in three dimensions will help us to better predict CME arrival times and impact angles at the Earth," says Dr Thomas Moran of the Catholic University, Washington, USA.
In collaboration with Dr Joseph Davila, of NASA's Goddard Space Flight Center, Greenbelt, USA, Moran has analysed two-dimensional images from the ESA/NASA Solar and Heliospheric Observatory (SOHO) in a new way to yield 3D images.
Their technique is able to reveal the complex and distorted magnetic fields that travel with the CME cloud and sometimes interact with Earth's own magnetic field, pouring tremendous amounts of energy into the space near Earth.
"These magnetic fields are invisible," Moran explains, "but since the CME gas is electrified, it spirals around the magnetic fields, tracing out their shapes." Therefore, a 3D view of the CME electrified gas (called a plasma) gives scientists valuable information on the structure and behaviour of the magnetic fields powering the CME.
The new analysis technique for SOHO data determines the three-dimensional structure of a CME by taking a sequence of three SOHO Large Angle and Spectrometric Coronagraph (LASCO) images through various polarisers, at different angles.
Whilst the light emitted by the Sun is not polarised, once it is scattered off electrons in the CME plasma it takes up some polarisation. This means that the electric fields of some of the scattered light are forced to oscillate in certain directions, whereas the electric field in the light emitted by the Sun is free to oscillate in all directions.
Moran and Davila knew that light from CME structures closer to the plane of the Sun (as seen on the LASCO images) had to be more polarised than light from structures farther from that plane. Thus, by computing the ratio of polarised to unpolarised light for each CME structure, they could measure its distance from the plane. This provided the missing third dimension to the LASCO images.
With this technique, the team has confirmed that the structure of CMEs directed towards Earth is an expanding arcade of loops, rather than a bubble or rope-like structure.
Although this technique had been independently developed previously to study relatively static structures in the solar atmosphere during eclipses, this is the first time that it is applied to fast moving CMEs.
Moran and Davila believe that their method will complement data from the upcoming NASA's Solar Terrestrial Relations Observatory (STEREO) mission, scheduled for launch in February 2006. STEREO will use two widely separated spacecraft to construct 3D views of CMEs by combining images from the different vantage points of the twin spacecraft.
Commenting on this result, Bernhard Fleck, SOHO Project Scientist at ESA, said: "These are really amazing images. Once again scientists have come up with a clever idea for analysing SOHO data in ways that were not even dreamt of when the mission was designed."
3 stills from the movie
This movie shows a 3D rendering of the data in Figure 2. It starts out viewing the Sun from SOHO's perspective, then rotates the scene to view the data from the side, and finally from the top. Note that one distinct feature shown at about 11 o'clock in Figure 2 panel a has been left out of the movie, because it is a static structure and not a part of the CME.
Notes to Editors:
This new result by T. Moran and J. Davila is published in today's issue of the magazine Science.
More about SOHO
SOHO is a project of international co-operation between ESA and NASA to study the Sun, from its deep core to the outer corona, and the solar wind. Fourteen European countries, led by the European Space Agency and prime contractor Astrium (formerly Matra-Marconi), built the SOHO spacecraft. It carries twelve instruments (nine European-led and three American-led) and was launched by an NASA's Atlas II-AS rocket on 2 December 1995. Mission operations are co-ordinated at NASA's Goddard Space Flight Centre. The spacecraft was designed for a two-year mission but its spectacular success has led to two extensions of the mission, the first until 2003, and then again until March 2007.
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