Max-Planck-Institut für Sonnensystemforschung

The solar cycle is the magnetic cycle of the Sun. Inorder to understand the solar cycle and its properties, we need to understandhow the Sun is generating its own magnetic fields and organizing them. The FluxTransport Dynamo model is the most successful model to understand the solarmagnetic field generation process. But it has some inherent limitations. Inmost of the dynamo models, a single cell meridional circulation is used butthere is some recent observational evidence that the meridional circulation ofthe Sun may not have a single cell structure rather it might have a double cellor multi-cell structure. I shall discuss that the new observations are notimposing any serious threat and our model works perfectly fine. Many processesin this models are inherently 3D. In 2D we can model them very crudely by usingsome simple parametric form. So I shall explain the next generation 3D Flux TransportDynamo model which will be more realistic. The build up of solar polar fieldsusing this model will also be discussed. Apart from that I shall discuss thathow inclusion of observed high resolution non-axisymmetric convective flows inour model from SDO Dopplergram data affects the behaviour of theBabcock-Leighton process and helps us to put a better constraint on the surfacediffusivity. [mehr]

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Helicity density (hereafter helicity), representing local structural properties of turbulence, is known to play important roles in the global magnetic-field induction (dynamos). In this talk, special emphasis is put on the role of helicity in the momentum transport. In addition to the usual eddy viscosity (represented by the turbulent energy), in the presence of breaking symmetry, a finite turbulent helicity effect enters the Reynolds stress. Recent developments on this line of studies are reported, which include the analysis of the stress equation, subgrid-scale (SGS) modelling. A possible application of this effect to the angular-momentum transport in the solar convection zone, as well as the extension to the magnetohydrodynamic (MHD) case, will be discussed. [mehr]

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The ubiquitous thin current sheets in high-Lundquist-number space and astrophysical plasmas are known to be unstable to the plasmoid instability, which disrupts current sheets to form smaller structures such as flux ropes and secondary current sheets. The plasmoid instability thus plays a significant role in magnetic reconnection and magnetohydrodynamic (MHD) turbulence. In this talk, I will discuss the role of the plasmoid instability in triggering the transition from slow to fast reconnection. Then I will talk about the interplay between plasmoid instability and MHD turbulence, including self-generated 3D turbulent reconnection mediated by the plasmoid instability, as well as more recent results on plasmoid-mediated energy cascade in MHD turbulence. Finally, I will also present some evidence of the plasmoid instability in solar observations, investigated in collaboration with the Max Planck Institute for Solar System Research, which is made possible by the Max Planck Princeton Center. [mehr]