Testing for Dynamo Action
David W. Hughes · Cambridge University Press eBooks · 1994
It is important to determine whether a cosmical magnetic field is a consequence of dynamo action or, alternatively, is a slowly decaying fossil field. Similarly, in numerical simulations of magnetohydrodynamic turbulence we should like to distinguish between a dynamo-generated magnetic field and one that is simply decaying, albeit slowly. Here certain criteria are presented that must be satisfied before any positive claims can be made for dynamo action. INTRODUCTION Given the existence of a naturally occurring magnetic field, be it astrophysical or geophysical, it is natural to ask whether the field is generated by dynamo action or if instead it is a fossil field, trapped in the body since its formation. In certain contexts it is possible to give a definitive answer. For example, the Ohmic diffusion time of the Earth's core is of the order of 10 years whereas paleomagnetic records show that the magnetic field of the Earth has existed for 109 years. Consequently, since the field has been maintained for so many Ohmic decay times it must be generated by some sort of dynamo process. For astrophysical bodies on the other hand, for which typically the Ohmic time is comparable to the lifetime of the body itself, it is not so straightforward to assert that a field is dynamo-generated. Of course, there may be other factors suggesting the origin of the field, but simply on the basis of the Ohmic decay time the issue often cannot be decided. What we would like therefore is a test to distinguish between these two possibilities.