31st March 1998


Solar physicists at the University of St Andrews are combining theory with results from the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) to solve a long- standing mystery about the Sun - why is the solar corona so hot? According to Professor Eric Priest and his colleagues, energy from the Sun's magnetic surface is channelled into the corona by a process called 'magnetic reconnection'. In the corona, the magnetic field is broken down, releasing heat and raising the temperature of the gas to millions of degrees. And using SOHO observations they have also been able to show that there is little evidence so far to support a competing theory.

This new work will be included by Professor Priest in his talk, 'A startling new Sun', at the National Astronomy Meeting at the University of St Andrews on Wednesday 1st April.

The surface of the Sun has a temperature of only 6000 degrees kelvin (5730 degrees C), but its outermost layers of tenuous gas - the corona, which is visible at a total solar eclipse - is surprisingly very much hotter. Its temperature is several million degrees. How the corona is heated represents one of the most important unsolved mysteries in astrophysics which has tantalized solar physicists for the past 40 years.

"But the coronal heating problem is a really tough and complex one to tackle" says Professor Priest. "The corona consists of several types of structure which may be heated by different mechanisms. There are huge magnetic loops arching high above the solar surface, tiny intense cores of emission called X-ray bright points, and dark regions, called coronal holes, where the nature of the magnetic field allows hot gas (plasma) to stream out into interplanetary space."

Two main theories have been proposed to explain the high temperature of the solar corona. One of them involves magnetic waves travelling upwards from the surface of the Sun. Like water waves, magnetic waves carry energy. The other, the 'magnetic reconnection' theory, involves the generation of intense electric currents to discharge the energy directly into the corona.

To test the wave theory, Dr Robert Walsh and Dr Jack Ireland at St Andrews used the Coronal Diagnostic Spectrometer (CDS) instrument on SOHO to search for magnetic waves with periods between 30 and 1000 seconds in an active region of the Sun's surface where the magnetic field is strong. The magnetic structure of the region was also mapped out using data from the Michelson Doppler Imager (MDI) instrument, also on SOHO. The results were startling: in the layers of gas nearest the visible surface of the Sun (the chromosphere), where the temperature is about 10,000 degrees kelvin, there are clear wave-like motions with periods of about 300 seconds and 600 seconds; further up (the transition region) where the temperature is 200,000 degrees kelvin the waves can also be seen. But by the time the one- million-degree corona is reached no such wave motions were detected. It appears that waves are travelling up some distance, but they are not getting far enough to heat the corona.

However, the St Andrews team discovered that intense coronal brightenings known as X-ray bright points are heated by magnetic reconnection. Observations from a rocket instrument called NIXT have shown that such bright points have a complex internal structure of interacting magnetic loops. This structure agrees very well with predictions made by Professor Priest, Dr Clare Parnell and Dr Sara Martin.

"Magnetic reconnection gives a unified explanation for many diverse observations from SOHO that all fall into place when viewed together" says Professor Priest. For example: - Recently, Karel Schrijver, Alan Title and colleagues at Lockheed Martin discovered from MDI observations that the solar surface consists of a "magnetic carpet", in which the magnetic structure is completely replenished every 40 hours. The mechanism for changing the magnetic connections so rapidly is the magnetic reconnection process. - With the SUMER instrument, rapid jets of plasma in explosive events have been discovered and these are also naturally produced by magnetic reconnection. - The discovery made with the CDS instrument of bright spots that have been called "blinkers" are an inevitable consequence of magnetic reconnection.

Said Professor Priest, "It is only now that we are beginning to analyse and digest the results from SOHO, but there are some amazing surprises that are revolutionising our understanding of the Sun - our closest star".

Images to support this story can be found at

The general SOHO Web site is at


The SOHO satellite was launched 2 years ago as a major joint project between the European Space Agency (ESA) and NASA, with ESA as the major partner.

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