Four invited and seven contributed papers were presented in this full-day session. The proceedings of the session can be conveniently divided into two parts: a first part dealing with the observational constraints and the physical environment in which planetary dynamos operate and a second part addressed to the theory of the dynamo process of the generation of magnetic fields by fluid motions.
The geophysical and astronomical evidence for the dynamical properties of the earth's core and for the coupling between core and mantle was reviewed by M. G. Rochester. The observed correlation between changes of the length of the day and the westward drift of the nondipole geomagnetic field indicate the importance of the core-mantle coupling process. But because of the uncertainty of the electrical conductivity in the lowermost mantle, it has not yet been possible to decide whether electromagnetic coupling is sufficiently strong to explain the observations or whether the inertial coupling mechanism, by bumps on the core-mantle boundary, is required, as proposed by Hide. The increasing accuracy of seismic and other geophysical measurements suggests that detailed information about the bumps and about other dynamically important properties of the core will become available in the near future. The discovery of the Slichter mode of the inner core and perhaps even the measurement of subseismic internal modes of the outer core would greatly improve our knowledge of the physical conditions in this part of the earth. The interpretation of the more accurate measurements will required improved earth's models and intimate cooperation between astronomers and geophysicists. Rochester expressed regret that the astronomical community has adopted the old model of Molodenskii instead of the recently improved models by Wahr and others, which are preferred by geophysicists. New results were also presented concerning the possible implications, both for antidynamo theorems and for the Proudman-Taylor theorem, of the finite compressibility of planetary liquid cores.