Foreshock Pressure Variations and Magnetopause Motion

The dayside magnetopause is constantly in motion over time scales ranging from 1 to 20 minutes. Proposed causes for this motion include bursts of reconnection (FTEs), the Kelvin-Helmholtz instability, intrinsic (ambient) pressure variations in the solar wind, and pressure variations generated by kinetic processes within the Earth's foreshock (pressure pulses and hot flow anomalies). Geosynchronous orbit provides an excellent platform to determine the relative amplitudes (but not necessarily the prevalence) of these proposed mechanisms. The frequent one-to-one correspondence of transient compressional signatures at geosynchronous orbit and foreshock pressure variations, and the greater range of pre-noon than post-noon magnetopause motion, indicate that pressure pulse generated within the Earth's foreshock are the predominant cause of dayside magnetopause motion. This conclusion is bolstered by simulations which show that FTEs produce only weak perturbations deep within the dayside magnetopause, that the Kelvin-Helmholtz instability is unlikely on the dayside magnetopause, and that intrinsic variations in the solar wind dynamic pressure generally drive motion with greater amplitudes on the post-noon magnetopause.