Why the sun's corona reaches temperatures of several million degrees Celsius, even though it emits its light at a comparatively moderate 6000 °C on its surface, is one of the great mysteries of solar physics.
A "hot" lead to explain this effect leads to an area of the sun's atmosphere below the corona, the so-called magnetic canopy, a layer in which magnetic fields are largely aligned parallel to the sun's surface. Here, sound and plasma waves (Alfvén waves) have approximately the same speed and can therefore easily transform into each other. As their frequency and propagation speed increase with the strength of the magnetic field, they could break through the sound barrier in sufficiently strong magnetic fields and thus cause a shock-like conversion of the magnetic energy of the plasma into heat. In an experiment with the molten alkali metal rubidium and pulsed high magnetic fields, as can be generated at the High Field Magnetic Laboratory Dresden (HLD) with maximum values of almost 100 Tesla, a team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has developed a laboratory model and confirmed the theoretically predicted behavior of these plasma waves experimentally for the first time.
The experiments in liquid rubidium showed that a new signal with halved frequency appeared from a field strength of 54 Tesla, which is typical for rubidium. This sudden doubling of the period was in perfect agreement with the theoretical predictions. The Alfvén waves had broken through the sound barrier for the first time.
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