In an astrophysical context, Alfvén waves are found in the Earth’s ionosphere, and are thought to play a crucial role not only in corona heating, but also in accelerating the solar wind, which is the stream of charged particles expelled by the Sun. They were initially studied in experiments involving liquid metal, and later in plasma-physics facilities, where they are used to heat plasmas in fusion experiments. These different types of waves include Alfvén waves, which were discovered in 1942 by the Swedish plasma physicist Hannes Alfvén. The tricky part is determining whether the heating stems from sudden changes in magnetic field structures in the hot solar plasma just below the corona, or from the dampening of different types of waves. Researchers have known for a long time that magnetic fields must be the main culprit in corona heating. Using the HZDR’s Dresden High Magnetic Field Laboratory, they showed that the speed of Alfvén waves within a sample of molten rubidium metal can indeed surpass the speed of sound, thus validating a key prediction from the “hot trail” theory. Researchers from the HZDR in Germany have now shed new light on this mystery by confirming the behaviour of Alfvén waves at high magnetic fields for the first time. Direct evidence for this behaviour has, however, been lacking. Current theories suggest that it stems from a “hot trail” that forms in the region just below the corona, where strong magnetic fields mean that plasma waves known as Alfvén waves can travel at the speed of sound. This “corona heating”, as it is known, is one of greatest mysteries of solar physics. Then, in the outer reaches of its atmosphere, it becomes hotter again, with temperatures in the solar corona reaching several million degrees.
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