Coupling between alfvén and slow magnetosonic waves in an inhomogeneous finite- β plasma—II. Eigenmode analysis of localized ballooning-interchange instability

Abstract

Ballooning-interchange instability of guided hydromagnetic waves is examined by performing a numerical eigenmode analysis along a model field-line. This instability is driven by a combined effect of the plasma pressure gradient and the field-line curvature and can be considered as a candidate of the excitation mechanism of storm-time Pc5 pulsations in the outer magnetosphere. The system of eigenmode equations derived in an accompanying paper describes the coupling between Alfvén and slow magnetosonic modes due to inhomogeneity. Such a coupling results in oscillations with hybrid properties between the two, for example, the transverse magnetic perturbation in the radial direction and the out-of-phase relation in perturbed plasma and magnetic pressures ; they are important characteristics of the storm-time Pc5 pulsation. Eigenvalues of the fundamental mode oscillation have a positive imaginary part, that is, the fundamental eigenmode is unstable. The oscillation frequency of this unstable mode is proportional to the azimutha wave number, and is comparable with the ion drift frequency at the equator. These results suggest that the ion drift mode plays an important role in the unstable mode. Although the broad distribution of eigenfunctions along the field-line and a somewhat small value of the oscillation frequency are not consistent with observations, such inconsistencies are due to the oversimplification adopted in the model of plasma and magnetic fields. In practice, high energy particles can be expected to play a decisive role in determining the properties of the pulsations in the magnetosphere. Hence, it is concluded that the ballooning-interchange instability is a strong candidate for the excitation mechanism of the storm-time Pc5 pulsation.