Dynamo Theory
Introduction
Dynamo action describes the sustained conversion of kinetic energy into magnetic energy throughout the bulk of an electrically conducting fluid. Dynamos are commonly invoked to explain the origin of magnetic fields in the universe. Initially, dynamo action was introduced by Sir Joseph Larmor in 1919 to explain the magnetization of sunspots. Since then it has grown into a substantial body of knowledge and literature. Despite the many technicalities the themes of dynamo theory are straightforward: what type of velocities are capable of  dynamo action, and what are the properties of the generated magnetic fields.
The basics
The simplest dynamo problem is the kinematic one in which the back reaction of the magnetic field on the velocity is neglected. In this limit the velocity can be specified a priory and the dynamo problem reduces to the solution of the induction equation.