Magnetohydrodynamic Simulation of Reconnection in Turbulent Astrophysical Plasmas

by Fabien Widmer
Doctoral thesis
Date of Examination:2016-07-19
Date of issue:2016-08-31
Advisor:Prof. Dr. Jörg Büchner
Referee:Prof. Dr. Jörg Büchner
Referee:Prof. Dr. Stefan Dreizler
Referee:Prof. Dr. Hardi Peter
Referee:Dr. Wolfram Pd Schmidt
Referee:Prof. Dr. Andreas Tilgner
Referee:Prof. Dr. Laurent Gizon
Persistent Address: http://dx.doi.org/10.53846/goediss-5838

 

 

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Abstract

English

Turbulence is ubiquitous at large-Reynolds-number astrophysical plasmas like in the Solar corona. In such environments, the turbulence is thought to enhance the energy conversion rate by magnetic reconnection above the classical model predictions. Since turbulence cannot be simulated together with the large scale behaviour of the plasma, magnetic reconnection is studied through the average properties of turbulence. A Reynolds-averaged turbulence model is explored in which turbulence is self-sustained and -generated by the large scales (mean-) field inhomogeneities. Employing that model, the influence of turbulence is investigated by large-scale MHD numerical simulations solving evolution equations of the energy and cross-helicity of the turbulence together with the MHD equations. Magnetic reconnection is found to be either rapidly enhanced or suppressed by turbulence depending on the turbulence timescale. If the turbulence timescale is self-consistently calculated, reconnection is always strongly enhanced. Since the solar corona bears strong guide magnetic fields perpendicular to the reconnecting magnetic fields, the influences of a strong guide field on turbulent reconnection is separately investigated. A slow down of reconnection, obtained in the presence of a finite guide field, can be understood by a finite residual helicity working against the enhancement of reconnection by the turbulence. The influence of turbulence on magnetic reconnection is further studied by means of high resolution simulations of plasmoid-unstable current sheets. These simulations revealed the importance of turbulence for reaching fast reconnection.
Keywords: Magnetic reconnection, Mean-field turbulence, Guide field, Subgrid-scale
 
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