From Planetary to Stellar Dynamos - An investigation of the origin and the variability of stellar magnetic fields by direct numerical simulations

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Most low-mass stars possess a magnetic field. Stellar magnetic fields may differ a lot in their field topology, their field strength and their time dependence. Magnetic fields of low-mass stars and planets are maintained by currents resulting from the motion of a conducting fluid (or gas) in their interiors. Thus, the huge variability of stellar magnetic fields is most probably related to differences in the internal structure and interior dynamics of stars. Vice versa, magnetic fields have a major influence on stellar dynamics and stellar evolution in almost all evolutionary states. The proposed work will contribute to an explanation for the origin and the huge variety of stellar magnetic fields. The complexity of the dynamo problem together with a vast range of spatial and temporal scales rendered its direct numerical treatment impossible for long time. Only recently, increasing computer power made global, direct numerical dynamo simulations feasible, in particular for the geodynamo. These models describe the complicated interaction of a convection-driven, conducting and incompressible fluid (liquid iron in the Earth’s outer core) with a magnetic field. However, for stellar dynamos the simplifying assumption of an incompressible fluid is no longer valid, gas is compressible.A recently developed numerical dynamo code takes compressibility into account. We will carry out a systematic parameter study of compressible dynamo simulations guided by well-known results from incompressible modelling. In this way we intend to point out and to explain systematic differences between compressible and incompressible models with respect to the geometry of magnetic fields, the field strength, the time dependence, the mechanism of field generation and the strength and the pattern of differential rotation. A further comparison with observations along these lines will highlight the relevance of compressible effects and will offer insight into the working of stellar dynamos.