Magnetic fields of low-mass stars and planets are thought to originate from self-excited dynamo action in their convective interiors. The accepted theory, known as dynamo theory, describes the transfer from kinetic to magnetic energy as an instability process. Above a given threshold electrical currents, and thus magnetic fields, are amplified by a turbulent flow of a conducting fluid.
  Observations of the magnetic fields produced by direct numerical simulations (DNS) of dynamo action appear to fall into two categories: fields dominated by large-scale dipoles (such as the Earth and a fully convective star), and fields with smaller-scale and non-axisymmetric structures (such as the Sun). Two kinds of different temporal behaviour have also been identified: very irregular polarity reversals (as in the Earth), and quasi-periodic reversals (as in the Sun). Since the Earth and the Sun provide the largest database of magnetic field observations, these objects have been well studied and described in terms of alternative physical mechanisms: the geodynamo involves a steady branch of the dynamo equations, with fluctuations and possibly polarity reversals, whereas the solar dynamo takes the form of a propagating dynamo wave. The signature of this wave at the Sun's surface yields the well-known butterfly-diagram (Sunspots preferentially emerge at a latitude that is decreasing with time during the solar cycle).