Abstract. A numerical model of Pi 2 disturbances in the magnetosphere is developed. The model of the magnetosphere is the same as one previously used for the study of Pc 5 pulsations. It is semicylindrical with field lines being semicircular arcs and longitude measured parallel to the axis. The model magnetosphere is excited by a source on the boundary which is limited both in time and in longitude. The disturbance is then a superposition of modes corresponding to all the azimuthal wave numbers excited by the source. The computations show that a global oscillation is excited and that there can be a significant signal on the dayside at low latitudes. The role of the plasmasphere in producing a low-latitude response is important. Because of the simplifications of the model the most important aspect of the calculations is the qualitative understanding that they supply. Nevertheless, quite good quantitative agreement with observation is obtained. The calculations support earlier suggestions about the response of the magnetosphere to the initiation of the substorm current wedge.
There is a rich variety of observations of long‐period pulsations in the ring current region. One class of such pulsations is characterized by large azimuthal wave numbers and close confinement to a field line. In this paper, computations are made of the eigenfrequencies and fields of standing oscillations which can exist in a realistic self‐consistent magnetohydrodynamic model of such a region. The model, which depends on a few simple parameters, allows for the shape of the field line to be distorted from a dipole. In this paper it is limited to isotropic pressure. The magnetic field and transverse gradient of pressure can be defined. It is assumed that growth and loss processes are small and in balance because the plasma has reached a quasi‐linear equilibrium state. The computations show that a rich variety of compressional modes can exist. They are much more complex than simple coupled Alfvén and magnetosonic modes which are often invoked in such circumstances. At some points along the field line, such modes may be closely coupled while at other points they are essentially uncoupled with different wavelengths. This leads to modes with complex nonharmonic structures. It is noted that the transverse pressure gradient allows an MHD instability, related to the flute instability, to exist in appropriate circumstances. Such calculations should allow good comparison of theory and observation.
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