Characteristics of Diamagnetic Hydromagnetic Waves in a High-beta Model Magnetosphere

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A linear eigenmode analysis of drift Alfven wave is performed in a curved magnetic field configuration in a high- <beta> plasma to find a generation mechanism of diamagnetic stormtime Pc5 pulsation characterized by a large azimuthal mode number (m>50). A magnetic field line obtained from a self-consistent model of magnetic field and plasma pressure is tied to a conducting ionosphere. An eigenmode equation is solved along the field line in order to obtain the eigenfrequency and eigenmode structure of the drift Alfven wave. In the absence of the coupling to the drift wave all obtained modes (poloidal oscillations modified by high- <beta> effect) are stable or weakly damped oscillations except for the fundamental mode. The fundamental mode becomes unstable to the ballooning instability driven by the unfavorable magnetic field curvature and is an aperiodic oscillation with zero real frequency. The eigenmode of the unstable mode is confined near the equator and the plasma pressure and magne … More tic pressure are almost out of phase. When the unstable mode is coupled to the drift wave, the unstable mode has a real frequency determined by a diamagnetic drift speed. The detailed results of the linear eigenmode analysis for both the ballooning interchange and drift Alfven modes in a model magnetosphere are described, and their implications to the origin of the stormtime pulsations are discussed. The obtained wave period of 270 sec for the drift Alfven mode with the azimuthal mode number m=50, its westward propagation and diamagnetic relationship between the perturbed magnetic and gas pressures suggest us that the unstable drift Alfven wave is the strong candidate for the observed stormtime Pc5 pulsations. By applying the adiabatic particle picture with a finite Larmor radius effect, we have extended our analysis to include a linear mode coupling between the shear (drift) Alfven and the slow magnetosonic waves under the curved field line configuration. Numerical solutions for such coupling oscillations in a model ring current region yield a fundamental mode of standing oscillations along a curved field line with a linear growth rate much larger than the real frequency of the average ion drift motion. It has been shown that the finite Larmor radius effect in the drift motion of ions has a stabilizing tendency as the perpendicular wavelength becomes short. Less