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Laboratory Experiments on Magnetic Phenomena in Geo- and Astrophysics

Periodic Reporting for period 2 - LEMAP (Laboratory Experiments on Magnetic Phenomena in Geo- and Astrophysics)

Reporting period: 2020-05-01 to 2021-10-31

How is the geodynamo powered, what drove the ancient lunar dynamo? Is it thermal or compositional buoyancy alone, as often assumed, or does precession play some complementary role? How do central objects, like protostars or supermassive black holes in the centres of galaxies, form from the gas discs surrounding them? For which disk parameters does the magnetorotational instability (MRI) enable angular momentum transport and allow for mass accretion, and where does the destabilizing field actually come from? Are there non-linear dynamos in accretion disks in which MRI and dynamo effect bootstrap each other? And, closely related to this: is the solar magnetic field really produced by a classical flux-transport dynamo, or is it also non-linear from the very outset, as proposed in the Tayler-Spruit dynamo model? Why is the butterfly diagram of sun-spots restricted to the equator-near strip around the solar equator, where the angular velocity at the tachocline just increases outward? Is that region indeed as stable as it has long been thought to be?
Much work has been devoted during the last decades to understand such intriguing problems of geo- and astrophysical magnetohydrodynamics (MHD). Powerful theories have been established and tested against observational evidence, millions of CPU-hours on the most advanced parallel computers in the world have been spent to gain numerical insight into the complex interaction of flows and magnetic fields. But only recently it has become possible to tackle those fundamental processes in dedicated, paradigmatic liquid metal experiments. After many years of preparations, the Riga and the Karlsruhe dynamo experiments became operative at the end of 1999 and proved magnetic-field self-excitation in large scale flows of liquid sodium. Starting in 2006, self-excitation and most interesting dynamical effects, such as geodynamo-like reversals and excursions, were observed in the von Kármán sodium experiment (VKS) in Cadarache. Also in 2006, we obtained first experimental evidence of the helical version of the MRI in the Potsdam ROssendorf Magnetic InStability Experiment (PROMISE). Meanwhile, the azimuthal MRI has also been observed at an enhanced PROMISE set-up, and the current-driven, kink type Tayler instability (TI) was demonstrated in a long column of a liquid metal, just by running a strong electrical current through it.
With the present project, we plan to make three further breakthroughs in the field. First, we will attack experimentally the long-standing problem of whether precession is a viable driver of planetary dynamos. Second, we will run an experiment in which various combinations of MRI and TI can be studied, with the main goal of proving the standard version of MRI with only an axial magnetic field being applied. Third, we will investigate the question whether rotational flows with a radially increasing angular velocity, such as it prevails in an equator-near-strip of the solar tachocline, might be destabilized by magnetic fields.
All those experiments will be carried out in the framework of the large scale liquid sodium platform DRESDYN (DREsden Sodium facility for DYNamo and thermohydraulic studies: A European Platform) at HZDR, whose building construction has been finalized in 2017.
During the reporting period, the project was concerned with 1) Numerical simulations and water experiments with the aim to identify the optimum parameter range for dynamo action in the precession driven flow, 2) Numerical simulations and experiments on the magnetorotational instability (MRI) for rotating flows with negative shear, 3) Numerical simulations and predictions for magnetorotational instability for rotating flows with positive shear, such as Super-AMRI and Super-HMRI. Up to now, the project has resulted in fifteen papers in refereed journals, one of which, devoted to the synchronization of the Solar dynamo (Stefani et al., Solar Physics 294, 60 (2019)), has received enormous public awareness (Altmetric score of 221) with press coverage in “New Scientist”, “EOS” and “Newsweek”. Another surprising finding (Mamatsashvili et al., Physical Review Fluids 4, 103905 (2019), Altmetric score 94) was that flows with weak positive shear, such as in the near-equator parts of the solar tachocline, can be destabilized by Super-HMRI.
Until the end of the project, we plan to run a large-scale liquid sodium experiment on precession driven dynamo action, and another experiment on the combined action of magnetorotational and Tayler instability.
The DRESDYN precession driven dynamo experiment: some impressions from the construction