A study of geomagnetic micropulsations
The general properties of micropulsations are reviewed, together with some theories of their origin. A particular study is made of the hydromagnetic wave theory and of the probability of these waves being produced by gyrating charged particles in the magnetosphere. The conditions are discussed under which gyration-induced hydromagnetic waves have frequencies within the micropulsation range. Assuming the production of Alfvén waves by some process in the magnetosphere, a layer model is developed to investigate the transmission of these waves down along the Earth’s magnetic field lines to the ionosphere. One feature of this model is a realistic ionospheric termination, in which a phase shift and amplitude reduction may be introduced into back reflected waves. Calculations using the model indicate a definite harmonic structure associated with micropulsations and only a small variation of frequency with geomagnetic latitude; they also provide a good explanation of the frequency structure and diurnal variation of ‘pearls’.