A LINEAR PERTURBATION ANALYSIS OF MAGNETOPAUSE MOTION IN THE NEWTON-BUSEMANN LIMIT

The response of the magnetopause surface to time-varying solar wind dy namic pressure is examined. We argue that to a first approximation the magnetopause surface may be considered as analogous to an elastic mem brane. Upon displacement from equilibrium resulting from a change in a pplied external pressure, it moves to a new equilibrium under the equa tion of motion of a forced, damped, simple harmonic oscillator. We der ive this equation of motion by linearising for small perturbations the momentum equation for flow past a nonrigid ellipsoidal body in the Ne wton-Busemann limit. Though our approach is only an approximation to t he real dynamics of the magnetopause boundary, it serves to demonstrat e the importance of inertia in the system response. It allows us to es timate the natural eigenperiod of magnetopause oscillation as typicall y around 7 min, the precise value depending on solar wind conditions, However, the magnetopause eigenoscillation is furthermore found to be strongly damped, regardless of solar wind conditions. One consequence of these properties is that short-period fluctuations in the solar win d dynamic pressure elicit a suppressed magnetospheric response. We out line other theoretical expectations by which our model may be tested a gainst observation, and discuss the implications of our findings for c urrent interpretations of spacecraft observations made in the dynamic magnetopause environment.