Soft Matter and Complex Systems Seminar

It is well known that a field of random waves in a fluid of non-zero resistivity is capable of exciting a large-scale magnetic field through creation of an electromotive force (EMF) which leads to exponential growth of magnetic energy until the growing Lorentz force reacts back upon the wave field, leading to a saturated state. For highly conducting plasma it is generally found that kinematic fast-dynamos with finite growth rate in the limit of vanishing resistivity, have a pathological structure, non-differentiable wherever they are non-zero; the applicability of fast-dynamo theory to natural physical systems is then questionable.
Here we relax the standard simplifying assumptions of stationarity and homogeneity of the background turbulence and introduce new fast-dynamo mechanisms, fully dynamic, that is incorporating the back reaction of the Lorentz force on the flow (hitherto scarcely considered), for which the growing magnetic field remains smooth during the whole dynamo process. This results from a random superposition of waves, perturbed by the magnetic field. Particularly effective are the interactions of ‘beating’ waves (close-frequency waves) and nonlinear effects in the mean electromotive force leading to very fast amplification of the mean magnetic field. The renormalization approach is udertaken to obtain final mean-field equations and saturation of the large-scale field.
The theory has the potential to be applied to the dynamo generation of magnetic fields in the ionised gas of the early universe, both before and during the process of galaxy formation. In such a plasma, the resistivity is extremely low, giving characteristic diffusion times many orders of magnitude greater than the age of the entire universe and hence negligible. Nevertheless the large-scale galactic magnetic fields and fields of galaxy clusters are observed, thus non-resistive dynamo mechanisms are strongly desirable in this context.

It is well known that a field of random waves in a fluid of non-zero resistivity is capable of exciting a large-scale magnetic field through creation of an electromotive force (EMF) which leads to exponential growth of magnetic energy until the growing Lorentz force reacts back upon the wave field, leading to a saturated state. For highly conducting plasma it is generally found that kinematic fast-dynamos with finite growth rate in the limit of vanishing resistivity, have a pathological structure, non-differentiable wherever they are non-zero; the applicability of fast-dynamo theory to natural physical systems is then questionable.
Here we relax the standard simplifying assumptions of stationarity and homogeneity of the background turbulence and introduce new fast-dynamo mechanisms, fully dynamic, that is incorporating the back reaction of the Lorentz force on the flow (hitherto scarcely considered), for which the growing magnetic field remains smooth during the whole dynamo process. This results from a random superposition of waves, perturbed by the magnetic field. Particularly effective are the interactions of ‘beating’ waves (close-frequency waves) and nonlinear effects in the mean electromotive force leading to very fast amplification of the mean magnetic field. The renormalization approach is udertaken to obtain final mean-field equations and saturation of the large-scale field.
The theory has the potential to be applied to the dynamo generation of magnetic fields in the ionised gas of the early universe, both before and during the process of galaxy formation. In such a plasma, the resistivity is extremely low, giving characteristic diffusion times many orders of magnitude greater than the age of the entire universe and hence negligible. Nevertheless the large-scale galactic magnetic fields and fields of galaxy clusters are observed, thus non-resistive dynamo mechanisms are strongly desirable in this context.