Scientists find solution to solar puzzle

March 23, 2009

Scientists from the University of Sheffield and Queen's University Belfast have made a unique discovery which will help us understand one of the most puzzling features of the Sun. The research has helped explain why the outside atmosphere of the Sun is actually hotter than the inner photosphere.

The surface of the Sun, known as the photosphere, reaches temperatures of 5,000 degrees. It would seem logical that the temperature is lower further away from the Sun; however the outer atmosphere, known as the corona, can reach temperatures of over a million degrees.

The study, which has been published in the illustrious Science journal, has discovered evidence for the existence of a new breed of solar wave, called the Alfvén wave, which transports energy to heat the Sun's corona. This phenomenon was first proposed by Hannes Alfvén in 1942, but despite winning a Nobel Prize for his pioneering work in this field, hard evidence for the wave has not been found until now in the solar atmosphere.

The team used data from the Swedish Solar Telescope in the Canary Islands, to detect for the first time these waves in the Sun's lower atmosphere. This telescope is the largest and most powerful solar telescope in Europe and is able to produce some of the sharpest images currently available.

Professor Robert von Fay-Siebenburgen, from the University of Sheffield's Department of Applied Mathematics, said: "The heat was on to find evidence for the existence of Alfvén waves. International space agencies have invested considerable resources trying to find purely magnetic oscillations of plasmas in space, particularly in the Sun. These waves, once detected, can be used to determine the physical conditions in the invisible regions of the Sun and other stars."

Dr Daivd Jess, lead researcher on the project from Queen's University Belfast, said: "Often, waves can be visualized by the rippling of water when a stone is dropped into a pond, or by the motions of a guitar string when plucked. However, Alfvén waves cannot be seen so easily. In fact, they are completely invisible to the naked eye. Only by examining the motions of structures and their corresponding velocities in the Sun's turbulent atmosphere could we find, for the first time, the presence of these elusive Alfvén waves."

Professor Mihalis Mathioudakis, the leader of the Queen's University Belfast Solar Group, said: "Understanding solar activity and its influence on the Earth's climate is of paramount importance for human kind. The Sun is not as 'quiet' as many people think. The solar corona, visible from Earth only during a total solar eclipse, is a very dynamic environment which can erupt suddenly, releasing more energy than 10 billion atomic bombs. Our study makes a major advancement in the understanding of how the million-degree corona manages to achieve this feat."

University of Sheffield

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