Excitation of magnetohydrodynamic cavities in the magnetosphere

Abstract

Abstract The normal modes of the magnetosphere can be excited by disturbances incident from the solar wind. These normal modes take the form of cavity resonances which may in turn excite field line resonances. They are often observed as ULF pulsations. There has been much interest in modelling such disturbances using a box model of the magnetosphere. The cavity is then treated either like a box, or like a waveguide. This paper shows how such a cavity or waveguide, with a leaky boundary representing the magnetopause, is excited by an incident signal which is expressed as a space- and time-limited Fourier synthesis of plane waves. Using WKB solutions the reflection and transmission coefficients of the cavity are found. It is shown from these that, in general, the oncoming signal is reflected. At frequencies corresponding to the normal modes, however, the energy is captured and transmitted down the waveguide. The nature of this process is clarified and discussed. In a simple model the waveguide modes correspond to poles of the frequency response in the complex frequency plane. The behaviour of these modes is calculated and the dependence of their frequency and damping on magnetospheric parameters is computed.