We use geodynamo models and experimental dynamos to gain insight into likelymechanisms of secular variation. These include zonal flows, hydromagneticwaves, flux expulsion and magnetic diffusion. We aim to test such mechanismsusing improved time-dependent field models derived from geomagneticobservations.
The problem of the interaction of small‐amplitude plane hydromagnetic waves with plane, oblique hydromagnetic shocks is treated. When the perturbations of the flow field on either side of the shock are expanded in terms of incident and diverging eigenwaves, the linearized de Hoffmann‐Teller relations across the shock yield a system of equations for the amplitudes of the diverging waves and the motion of the perturbed shock. The shock motion and the amplitudes of the diverging waves as a function of the amplitude and direction of any given incident wave are found using this system of seven linear, inhomogeneous equations, along with Snell's laws. Snell's laws are interpreted graphically by making use of the geometry of the hydromagnetic wave normal surfaces in a moving medium. By considering some special cases it is shown that hydromagnetic waves are amplified on passage through a fast shock. The turbulent character of the region between the bow shock of the earth and the magnetopause may be due in part to the amplification of turbulent waves in the solar wind on passing through the bow shock.
The present paper deals with some of the areas of current researchinterest in the physics of low-frequency waves in the earth'smagnetosphere. Particular attention is given to variations known asgeomagnetic pulsations. These variations, observed on the ground and onspacecraft, are presently recognized as originating from a plasmaphenomenon - the hydromagnetic waves that occur in the magnetosphere. Asa result, the pulsations can be thought of as the lowest frequency wavesthat can be sustained in the magnetospheric plasma. Some experimentaltechniques which have been used to study the hydromagnetic waves areexamined.