Radiative transfer dynamo effect

Magnetic fields in rotating and radiating astrophysical plasma can be produced due to a radiative interaction between plasma layers moving relative to each other. The efficiency of current drive, and with it the associated dynamo effect, is considered in a number of limits. It is shown here, however, that predictions for these generated magnetic fields can be significantly higher when kinetic effects, previously neglected, are taken into account.

Figure 1

Parallel radiating and absorbing slabs of plasma, immersed in different magnetic fields at different temperatures, in relative parallel motion.

Figure 2

Schematic picture of the magnetic field growth. Magnetic field grows approximately linearly with time until it saturates at equilibrium value $B_{\mathrm{eq}}$. Kinetic effects increase $B_{\mathrm{eq}}$ to the same extent as they increase the current drive efficiency, but they hardly change the time to reach equilibrium $t_{\mathrm{eq}}$. The actual value of the magnetic seed is determined at some characteristic time $t_{\mathrm{seed}}\ll t_{\mathrm{eq}}$, and it increases to the same extent as the efficiency increases.

Figure 3

(a) ${\omega }_{c1}\simeq {\omega }_{c2}$: electrons of slab 2 with negative velocities around $-\overline{\beta }$ (left blue region) interact with the large number of electrons of slab 1 (left red region), while symmetric electrons of slab 2 with positive velocities around $\overline{\beta }$ (right blue region) interact with small number of electrons of slab 1 (right red region). (b) ${\omega }_{c1}\ne {\omega }_{c2}$: electrons of slab 2 with negative velocities around $-\overline{\beta }$ (left blue region) has large number of electrons of slab 1 in the nonabsorption window (left white region) and thus absorb less energy than symmetric electrons of slab 2 with positive velocities around $\overline{\beta }$ (right blue region), which have small number of electrons of slab 1 in the nonabsorption window (right white region).

Figure 4

Absorbed power density per electron as a function of parallel velocity for ${\beta }_{\mathrm{th}}=0.01, \overline{\beta }=0.05$ and different values of $a$: $a=0.01$ (solid blue), $a=0.1$ (dotted red), $a=1.0$ (dashed orange), $a=3.0$ (dash-dotted green).