Propagation of a sudden impulse through the magnetosphere initiating magnetospheric Pc5 pulsations

摘要

We compare multipoint observations of an interplanetary shock’s interaction with the Earth’s magnetosphere on 29 July 2002 with results from global MHD simulations. The sudden impulse associated with the shock’s arrival initiates global ultralow‐frequency waves with periods from 2 to 5 min. We interpret four cycles of Bz oscillations with T = ～3 min at Geotail in the postdawn magnetosphere as radial magnetopause oscillations. GOES 8, in the same late morning sector, observed compressional and toroidal waves with the same frequency at the same time. GOES 10, in the early morning sector, observed toroidal waves with a slightly lower period. We suggest that these observations confirm the mode coupling theory. The interplanetary shock initiates compressional magnetospheric waves which, according to our estimates, oscillate between the ionosphereand magnetopause and gradually convert their energy into that of standing Alfven waves. At the same time, Polar in the outer predawn magnetosphere observed strong velocity oscillations and weak magnetic field oscillations with a ～4 min period. Global MHD models successfully predict these oscillations and connect them to the Kelvin‐Helmholtz instability which results in large flow vortices with sizes of about ten Earth radii. However, the global models do not predict the multiple compressional oscillations with the observed periods and therefore cannot readily explain the GOES observations.