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Dr. Frank Stefani

Head Geo- and Astrophysics
f.stefaniAthzdr.de
Phone: +49 351 260 3069

Alfvén waves

The theoretical derivation of the existence of magnetohydrodynamic waves by Hannes Alfvén was recognized with the Nobel Prize for Physics in 1970. These Alfvén waves now play an important role in the physics of the solar atmosphere and magnetosphere as well as in fusion research. Soon after their prediction in 1942, Alfvén waves were first detected in liquid metal experiments and later studied in detail in complex plasma devices. While the speed of the Alfvén wave in liquid metals is usually significantly lower than the speed of sound, this relationship is reversed in plasma experiments. Conditions under which both speeds are equal have not yet been achieved in either type of experiment. However, this point in the solar atmosphere is crucial for heating the solar corona to temperatures of several million degrees. It is assumed that it is precisely here that sound waves are converted into Alfvén waves, which can then introduce sufficient "heating energy" into the corona.

This condition was achieved for the first time in an experiment at the Dresden High Magnetic Field Laboratory (HLD) using a pulsed field of up to 62 Tesla (Fig. 1). For liquid rubidium, the "magic point" corresponds to a field strength of 53 Tesla. By feeding an alternating current at the bottom of the container while simultaneously applying the pulsed magnetic field, Alfvén waves were generated in the melt, whose upward movement was measured at the expected speed. While up to a field strength of 53 Tesla all measurements were dominated by the frequency of the alternating current fed in, a new signal with a half frequency appeared at exactly this point (Fig. 2). This sudden period doubling was in perfect agreement with the theoretical predictions of a parametric resonance between sound and Alfvén waves.

Figure 1: Alfvén wave experiment at the HLD. (a) Stainless steel container for liquid rubidium. (b) Sample holder with 4 pick-up coils PP1-PP4 and 4 compensation coils (CC). (c) Details of the construction with the electric potential probes PP1-PP4 on the container. The orange triangles illustrate the three side electrical contacts (RC) for the AC power supply (LC denotes the associated lower contact). (d) Illustration of the direct drive of a torsional Alfvén wave by the azimuthally directed Lorentz force in the lower area of ​​the container.
Figure 2: (a) Time dependence of the pulsed magnetic field (top). The critical value of 54 Tesla (red line), at which the Alfvén speed is equal to the speed of sound, is exceeded in the time interval between approximately 40 and 65 ms. Electrical voltage measured at the lower contact (bottom) with a purely Ohmic component at the beginning and at the end of the experiment, and the electromotive force induced by the Alfvén wave. The two insets show signal details in the vicinity of 54 T, where the period-doubled 4 kHz signal begins and ends. (b) Spectral power density for the (impressed) 8 kHz component and the (period-doubled) 4 kHz components of the signal from (a), each for two different von-Hann windows of the Gabor transformation.

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