Solar Wind Control of Daytime, Midperiod Geomagnetic Pulsations

Numerous studies have established that various properties of geomagnetic pulsations are linked to various properties of the solar wind. The linkage in most cases is rather loose, suggesting that combinations of factors must be involved in generation and control of pulsation activity. We review briefly the most significant observational results and we describe and discuss critically the two most prominent models for external generation of magnetospheric waves. We present arguments favoring joint application of the models, wherein perturbations in the magnetosheath resulting from favorable interplanetary field orientation are delivered to the magnetopause, transferred directly into the subsolar magnetosphere, and amplified into surface waves on the flank of the magnetosphere by Kelvin-Helmholtz instability at high solar wind speed. This combination of circumstances can account for experimental correlations of pulsation occurrence with interplanetary field orientation, periods with interplanetary field strength, and amplitudes with solar wind velocity.