Dynamics of the magnetosphere

The impedance concept, so often used in solving problems in electromagnetic theory, is considered as it might apply to the dynamics of the magnetosphere. Geomagnetic ‘microstorms’ events usually given the label bps for ‘bay with pulsations and a sudden commencement’ are used in discussing the ‘transfer impedance’ of the magnetosphere. By this means we illustrate the relationship, in the frequency domain, between an assumed step-function change in the solar wind and an observed bps event. The transfer impedance so deduced is explained in terms of equivalent circuits. The circuit analogies are extended to include a unifying explanation of the origin of the micropulsations types pc and pt. Conclusions based on the application of statistical communication theory lead us to deduce possible mechanisms in the magnetosphere for the generation of micropulsation ‘signal’ and ‘noise.’ The signal arises in ‘cavity’ resonances of Alfvén waves both above and below the Alfvén wave velocity inversion and the noise in randomly distributed sources throughout the magnetosphere. Quasiperiodic modulation of micropulsations is attributed to the action of a random time distribution of discrete impulses, of solar wind velocity or density, in causing recurrent ‘ringing’ of the resonant cavities. The damping of pt signals can be explained on the basis of absorption in the ionosphere and the lower boundary of the Alfvén wave velocity inversion. Presumably these damping processes are not acting to the same extent in the day hemisphere as in the night. Some insight into the nature of the impulse response of the magnetosphere can be obtained at times of nuclear detonations in the upper atmosphere. This approach, though instructive, is limited because the forcing function is applied within rather than without the magnetosphere. However, the magnetograms at such times provide strong arguments in favor of the basic concepts introduced in this paper.