This chapter briefly reviews the theoretical explanations that have been put forward for the creation of cosmic magnetic fields (the dynamo) and their annihilation (magnetic reconnection) and examines the operation of these processes in both solar and planetary settings.

Many astrophysical bodies, including galaxies, stars, and planets, have an internally generated magnetic field. Although these bodies differ significantly in many aspects, they all possess within their interiors an electrically conducting fluid that is dominated by the Coriolis force because of their rapid rotation. In the case of the planets, the release of thermal and gravitational energy leads to convection in the planetary cores. In the case of stars and the Sun, convection is driven by heat from thermonuclear fusion. In many astronomical bodies the mean fields generated by the dynamo periodically reverse in time. A prominent example is the 22-year periodicity of the magnetic field of the Sun. To answer the question of the origin of magnetic fields, it is necessary to understand how magnetic fields are generated and maintained in rapidly rotating, convective fluids. This understanding is the goal of dynamo theory.

The dynamo process can be simply described as follows: a moving electrically conducting fluid stretches, twists, and folds the magnetic field. Dynamo action occurs if a small-amplitude seed magnetic field is sustained and amplified by the flow. The magnetic field increases in strength until the resultant magnetic forces are sufficient to feed back on the flow field. Dynamos can be quite complicated, and fundamental questions can be posed. How does a given flow generate a magnetic field? How does the generated magnetic field act to modify the flow? What energy source sustains the flow? While the first two questions can be studied within the context of magnetohydrodynamics, the answer to the last question depends on the specific physical system being studied. Finally, magnetic reconnection (in the generic sense of a mechanism that alters magnetic field topology) is an intrinsic part of any dynamo mechanism. The various magnetic field components that are generated by plasma flows must ultimately decouple and condense into a large-scale field (usually the dipole field in astronomical objects). The connectivity of field lines must change for this condensation to take place, which requires reconnection. What, therefore, are the processes that control magnetic reconnection in environments where dynamo action is important (e.g., the convection zone in the Sun or in the interior of planetary bodies)? In a self-consistent dynamo model, all these questions are related and so must be studied together.

Kinematic dynamo theory studies the generation of a magnetic field by a given flow. The importance of flow is described by the (nondimensional) magnetic Reynolds number R_m, defined as the ratio of magnetic diffusion time to the flow convection time. Dynamo action occurs if the growth rate of magnetic field perturbations is positive, that is, if the amplitude of an initially small perturbation increases with time. From kinematic theory the necessary condition for dynamo action is typically R_m ≥ 10. The physical significance of this condition is that the electromotive force associated with the flow has to overcome the magnetic dissipation in the fluid in order for a dynamo to occur. Another important result of kinematic dynamo studies is the demonstration that an axisymmetric magnetic field cannot be generated by an axisymmetric flow. This result implies that dynamo action must be three-dimensional.

When the magnetic Reynolds number R_m is large (i.e., indicates a faster flow, or less electrical resistivity in the fluid), the field lines are “frozen” in to the flow and are thus stretched, twisted, and bent (Figure 2.1). In order for the net flux to increase, the field lines must reconnect (alter their topology). Because magnetic diffusion is weak, field line reconnection takes place in regions of small spatial scale. Overall, the dynamo process generates new magnetic field lines and the magnetic flux increases with time. A major mystery is the source of magnetic diffusion required to change the field topology, which greatly exceeds that resulting from classical collisional processes.

Front Matter (R1-R10)

Executive Summary (1-4)

1 Our Local Cosmic Laboratory (5-10)

2 Creation and Annihilation of Magnetic Fields (11-27)

3 Formation of Structures and Transients (28-45)

4 Plasma Interactions (46-56)

5 Explosive Energy Conversion (57-64)

6 Energetic Particle Acceleration (65-76)

7 Concluding Thoughts (77-78)

Appendix A: Statement of Task (79-82)

Appendix B: Study Groups (83-84)

Appendix C: Acronyms and Abbreviations (85-86)