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Discovery of Alfvén waves in the corona of the sun

01 August 2011 KU Leuven

For the very first time, powerful Alfvén waves – magnetic plasma waves – have been observed in the sun’s corona. The plasma in the observed waves moves at speeds of 20 km/s and the waves themselves propagate at high speeds of 200 to 250 km/s. On Earth, these waves would cover the distance between Leuven and New York in approximately 30 s. The discovery of these powerful Alfvén waves is important because they may explain the high temperatures in the sun’s corona and the high speeds of solar wind. This has been demonstrated by research conducted by Professor Marcel Goossens of the Centre for Plasma Astrophysics at K.U.Leuven.

The existence of these was predicted by the Swedish scientist Hannes Alfvén in 1942. His prediction was initially received with scepticism but its great significance was later recognised. In 1970, H. Alfvén was awarded the Nobel Prize for Physics. The discovery of powerful Alfvén waves in the high atmosphere of the sun, which is now being reported in Nature, occurred with telescopes on board the Solar Dynamics Observatory – a NASA satellite for solar observation.

The corona is the outermost layer of the atmosphere of the sun, which we can only see as a ring of light during a total solar eclipse. The gas of the corona is ionised as a result of the high temperatures. Such hot, ionised gas is called plasma and has particular characteristics in the presence of magnetic fields. The whole corona is filled with magnetic fields of various strengths and forms. Plasma is glued to the magnetic field lines of these fields almost perfectly: when the plasma moves, the magnetic field lines move too and vice versa.

Professor Goossens compares Alfvén waves to transversal waves on taut wires: “One might conceive of the magnetic field lines as taut wires loaded with mass (plasma). If the wire is pulled off balance and then released, creating a catapult effect, the wire will move backwards and forwards as a result of the power of tension. This sideways (transversal) movement runs along the wire as a wave. These transversal waves are called Alfvén waves when they occur on magnetic field lines loaded with plasma.”

The photosphere of the sun – the sun’s surface – has a temperature of approximately 6,000 C, while in the corona, the temperature rises to 2-3 million C. This increase in temperature contradicts our intuitive expectation that temperature reduces the further we move away from the heat source. Until now, scientists were unable to provide an explanation for this phenomenon. Moreover, resulting from the high temperature in the corona, the sun cannot retain plasma through solar gravity and the plasma flows away like solar wind. When it reaches the Earth’s orbit, solar wind can reach speeds of 300 to 600 km per second. This raised the question of how nature can increase the speed of plasma to hundreds of km per second.

The Alfvén waves that have now been observed provide an answer to both questions: as a result of enormous plasma speeds, these Alfvén waves produce enough energy to heat the corona and to propel solar wind.

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