An empirical relationship between coronal mass ejection initial speed and solar wind dynamic pressure

Interplanetary shocks that precede coronal mass ejections (CMEs) are mainly responsible for sudden impulses, which are characterized by a simple step-like increase in the horizontal H component. Such a magnetic field change has been explained as a compression of the magnetosphere by the passage of a sudden increase in the solar wind dynamic pressure. Strong compression of the dayside magnetopause could cause geosynchronous satellites to be exposed to solar wind environments where large fluctuations of the interplanetary magnetic field and highly energetic particles are present. In this study, we chose 26 event pairs consisting of a type II burst/CME occurring in conjunction with a sudden commencement/sudden impulse (SC/SI) whose solar wind, and Earth magnetic field data are available. We then investigated relationships among three physical properties (kinetic energy, directional parameter, and speed) of near-Sun CMEs, solar wind dynamic pressure, and SC/SI amplitude. As a result, we found that (1) the CME speed is more highly correlated with SC/SI amplitude than its kinetic energy and direction parameter; (2) by adopting the empirical relationship between solar wind dynamic pressure and amplitude of symmetric H (a steplike increase in the horizontal H component at low latitude), we could derive an empirical formula for the relationship between solar wind dynamic pressure near the Earth and the CME speed; (3) the CME speed has a linear relationship with the difference of magnetopause locations derived by using the model of Shue et al. (1998) at the subsolar point before and after the shock arrivals; (4) a fast CME greater than 1600 km s(-1) could be a driver of the magnetopause crossing of a spacecraft at geosynchronous orbit. Our results show that the CME speed is an important parameter for early prediction of geosynchronous magnetopause crossing.