Speaker
Dr
Giuseppina Nigro
Description
Using the magnetohydrodynamic (MHD) description, we develop
a nonlinear dynamo model that couples the evolution of the
large scale magnetic field with turbulent dynamics of the
plasma at small scale by electromotive force (e.m.f.) in the
induction equation at large scale. The nonlinear behavior of
the plasma at small scale is described by using a MHD shell
model for velocity field and magnetic field fluctuations.
The shell model allow to study this problem in a large
parameter regime which characterizes the dynamo phenomenon
in many natural systems and which is beyond the power of
supercomputers at today. Under specific conditions of the
plasma turbulent state, the field fluctuations at small
scales are able to trigger the dynamo instability. We study
this transition considering the stability curve which shows
a strong decrease in the critical magnetic Reynolds number
for increasing inverse magnetic Prandlt number
$\textrm{Pm}^{-1}$ in the range $[10^{-6},1]$ and slows an
increase in the range $[1,10^{8}]$. We also obtain
hysteretic behavior across the dynamo boundary reveling the
subcritical nature of this transition. The system,
undergoing this transition, can reach different dynamo
regimes, depending on Reynolds numbers of the plasma flow.
This shows the critical role that the turbulence plays in
the dynamo phenomenon. In particular the model is able to
reproduce the dynamical situation in which the large-scale
magnetic field jumps between two states which represent the
opposite polarities of the magnetic field, reproducing the
magnetic reversals as observed in geomagnetic dynamo and in
the VKS experiments.
