WHY IS THE SOLAR CORONA HOT?
During the total solar eclipse on 11 August, millions of ordinary people will have a rare opportunity (weather permitting) to see for themselves the Sun's outer atmosphere, called the "corona". Amazingly, the temperature of this faintly glowing gas is a million degrees or more. Somehow, energy from the Sun heats the corona. Exactly how the process works remains something of a mystery, but on Thursday 12th August, Dr Philippa Browning of the University of Manchester Institute of Science and Technology (UMIST), will round up the current state of expert knowledge on the subject at the UK Solar Physics Meeting in Guernsey, Channel Islands.
"During a total eclipse, we can actually see that the corona is not at all smooth but contains a lot of structure - rays and streamers, for example", says Dr Browning. "However, what we can see with our eyes is only a small part of the story. The corona emits x-rays far more strongly than visible light. Fortunately for life on Earth, the x-rays do not penetrate the Earth's atmosphere. But it means that the corona is best studied from space."
X-ray images of the Sun from satellites such as SOHO and YOHKOH show, better than visible images, that the corona is highly structured and also very dynamic as a consequence of the strong magnetic field, which confines and controls the hot gas - and it is very hot gas indeed.
One of the most striking aspects of the corona is its temperature - around a million or more degrees - compared with the familiar yellow surface of the Sun which registers a much lower temperature of only a few thousand degrees. It is because of its very high temperature that the corona emits x-rays. No-one as yet fully understood why the corona is so hot. However, it is clear that the magnetic field plays a key role.
Solar experts now generally believe that energy from below the solar surface, carried by moving gas, is transmitted into the corona where it is stored as magnetic energy. Within the corona, this magnetic energy is then converted into heat. Some scientists take the view that the energy is carried by magnetic waves, others favour magnetic fields associated with steady electrical currents. As a result, the question "Why is the corona hot?" remains a topic of considerable controversy.
Relevant images of the Sun, and further links may be found on the web site for the SOHO solar space observatory, at: http://sohowww.nascom.nasa.gov/
CONTACT Dr Philippa Browning
CONTACTING THE PRESS ROOM AND PRESS OFFICERS AT THE UK SOLAR PHYSICS MEETING AND NATIONAL ASTRONOMY MEETING 9 - 13 AUGUST
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