Conditions for Instability

A dynamo at work in a radiative region derives its energy from the differential energy of rotation. The differential rotation itself results from several effects: (1) the change of the moment of inertia as evolution proceeds, (2) the spin-down due to some magnetic coupling and/or mass loss at the stellar surface, (3) the transport by meridional circulation.

Compared to the energy available for the solar dynamo driven by convection, the amount of energy available from differential rotation is limited. The energy density uB of a magnetic field of intensity B is uB = B2/ (8 n) (13.10). The Alfven frequency (13.18) is in a spherical geometry aA = B/ (4 np)1 rj. Thus, the magnetic energy density uB can be written as

0 0

Post a comment