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RAS PN 08/15 (NAM 6): Medical X-ray technique unveils the Sun's corona

Last Updated on Wednesday, 02 April 2008 09:27
Published on Tuesday, 01 April 2008 00:00
FRM_0002.gifA medical X-ray technique has been adapted to produce the first detailed map of the structure of the Sun’s outermost layer, the corona, created from direct observations. The results will be presented by Dr Huw Morgan at the RAS National Astronomy Meeting in Belfast on Tuesday 1st April.

Tomography, a technique with multiple medical applications including CAT scans, uses a series of images taken from many different angles to reconstruct a 3-dimensional map of a patient’s body.

"This is a breakthrough for scientists trying to understand the corona and the solar wind. We’ve been attempting to apply tomography to the solar corona for more than 30 years but it’s proved very difficult and very inaccurate until now. The new technique that I’ve developed is only in its infancy but shows great potential for areas of research like space weather," said Dr Morgan, of the University of Aberystwyth.

In applying tomography to the Sun, scientists have been faced by two major problems: firstly, they can’t see through the Sun to measure the corona on the far side, therefore nearly half their data is ‘missing’. Secondly, the outermost areas of the corona are more than a thousand times fainter than the regions near the Sun, which introduces huge potential errors to observations.

Dr Morgan has found a new way of processing coronal images, called Qualitative Solar Rotational Tomography (QSRT), to eliminate the steep drop in brightness and associated errors. He has then applied the technique to a series of images taken with the SOHO spacecraft’s LASCO instrument, so that the whole of the corona is mapped as the Sun’s rotation brings the ‘missing’ areas into view. The maps are 5 times more detailed than previous tomographical studies of the Sun and Morgan believes that they have the potential for 20 times better resolution in the future.

"I’ve now produced maps of the corona over almost a whole cycle of solar activity, so we can now see in unprecedented detail how structures develop and evolve in three-dimensions. The maps have produced some interesting results: for instance we’ve observed large areas of dense structures when the Sun is most active that are not predicted by current computer models. We’ve also found evidence that inner regions of the corona rotate at different speeds."

The technique is already being used by scientists at the Institute of Maths and Physics at Aberystwyth University to interpret their radio-wave observations of the solar wind. Dr Morgan, together with colleagues at the Institute of Astronomy at the University of Hawaii, is also using the maps to interpret ultraviolet observations of the corona.


Images and animations can be found at:



The RAS National Astronomy Meeting (NAM 2008) is hosted by Queen’s University Belfast. It is principally sponsored by the RAS and the STFC. NAM 2008 is being held together with the UK Solar Physics (UKSP) and Magnetosphere, Ionosphere and Solar-Terrestrial (MIST) spring meetings.


The corona is the Sun’s outer atmosphere and is the source of the solar wind. It is very faint and is only visible during a total solar eclipse or by using coronographs, which block out the bright disc of the Sun, or by looking at it in high-energy wavelengths. Temperatures in the corona reach 1 million degrees Celsius, but the source of this heating is not fully understood. The corona extends for 6 solar radii (4.17 billion kilometres) beyond the Sun’s visible surface (the chromosphere)

The solar wind is a stream of electrically-charged particles that is emitted in all directions from the Sun’s corona. It is made up of a roughly equal number of protons and electrons, with a few heavier ions. The average speed of the solar wind is 400 kilometres per second. High-speed solar wind emanates from coronal holes, regions where magnetic field lines loop out into space, allowing large quantities of solar atmosphere to escape.

Explosions at the Sun travel through space and often hit the Earth. These energetic magnetic clouds can disrupt communication, power supplies and be a major health hazard for astronauts and airline pilots. Understanding and predicting these storms is a major goal of solar science. The ability to map the whole 3D structure of the corona is a critical step towards achieving this goal.


SOHO (Solar & Heliospheric Observatory) is a joint ESA/NASA mission. SOHO’s orbit gives the spacecraft an uninterrupted view of the Sun. SOHO carries twelve instruments that monitor the Sun’s internal structure, its outer atmosphere and the solar wind. Launched in 1995, SOHO’s mission has now been extended until December 2009.


Dr Huw Morgan

Institute for Astronomy, University of Hawaii/ Institute of Maths. and Physical Sciences, University of Wales, Aberystwyth

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