Facilitating dynamo action via control of large-scale turbulence

The magnetohydrodynamic dynamo effect is considered to be the major cause of magnetic field generation in geo- and astrophysical systems. Recent experimental and numerical results show that turbulence constitutes an obstacle to dynamos; yet its role in this context is not totally clear. Via numerical simulations, we identify large-scale turbulent vortices with a detrimental effect on the amplification of the magnetic field in a geometry of experimental interest and propose a strategy for facilitating the dynamo instability by manipulating these detrimental “hidden” dynamics.

Figure 1

Cross section of the sphere, coordinate plane $yz$. The color represents the magnitude of the velocity field of the second SVD mode, ${\mathrm{Re}}_0=600$. The vortex has its strongest activity along the axis of symmetry, at a distance of $\sim$$0.14$.

Figure 2

Cross section of the sphere, coordinate plane $yz$. The color represents the $\varphi$ component of the velocity field of the second SVD mode, ${\mathrm{Re}}_0=600$. The color changes discontinuously from $y<0$ to $y>0$ due to the fact that the $y$ component of the unit vector ${\widehat{e}}_{\varphi }$ changes sign from $y<0$ to $y>0$.

Figure 3

Real and imaginary parts of the SVD temporal eigenfunction $v_1\left(t\right)$.

Figure 4

Real and imaginary parts of the SVD temporal eigenfunction $v_2\left(t\right)$.

Figure 5

Growth rates of $E_M$ for the runs “1”, “1+2”, and “1−2”, ${\mathrm{Re}}_0=600$.