Propagation of the Fast MHD Wave in the Earth Magnetosphere Generated by Sudden Impulses in the Solar Wind V. V. Shastun and O. Agapitov Astronomy and Space Physics department, National Taras Schevchenko University of Kyiv, Kyiv, Ukraine Abstract. Numerically, we studied the propagation properties of the fast MHD waves in the Earth magnetosphere caused by the fast interplanetary shock interaction with the magnetopause. The study of the temporal and spatial wave front peculiarities is based on the Huygens–Fresnel principle. A multispacecraft analysis of the propagation properties is performed to confirm the results of the numerical simulation. Obtained numerical results were found to be in a good agreement with the spacecraft observations. Introduction Interplanetary shocks (IPS) propagate in the solar wind to the Earth orbit with nearly constant speed. Main two sources of IPS are coronal mass ejection and corotating interaction regions. As they arrive near Earth orbit they interact with the magnetosphere, caused by a sudden increase of solar dynamic pressure. Stationary state corresponds to equilibrium of the solar dynamic pressure and the pressure of magnetosphere magnetic field. Thus, dayside magnetopause begins to shrink in the antisunward direction while equilibrium is being restored. Magnetospheric variations are often excited by an impulsive input such as interplanetary shocks and solar wind discontinuities (sudden impulses). This impact on the dayside magnetopause launches sudden perturbations into the magnetosphere. The propagation of the sudden impulses (SI) in the Earth magnetosphere is often assumed to be in a form of the fast MHD wave [Francis et al., 1959; Nishida, 1978]. During the propagation into the magnetosphere they continuously produce transverse waves via mode coupling by inhomogeneity or curved magnetic field geometry [Hasegawa et al., 1983; Lee and Hudson, 2001]. Recently, investigations of the propagation of SI have shown that compressional pulses can play an important role in the energization of the radiation belt electrons and protons [Li et al., 1993; Hudson et al., 1995, 1996]. Existence of the ionosphere influences the signal arrival at the Earth’s surface. Kikuchi and Araki [Kikuchi and Araki, 1979] studied the ionosphere responses associated with the SI assuming the Earth-ionosphere waveguide system. The time distributions onset and their observation characteristics have been investigated and well summarized by Araki [Araki, 1977]. Recently, the MHD wave propagation and coupling have been investigated in the magnetotail by adopting a waveguide [Allan and Wright, 1998]. The propagation properties of SI have been performed by simultaneous ground based observations and satellite measurements [Nopper et al., 1982; Wilken et al., 1982; Nagano and Araki, 1984]. Still there exist many questions about the propagation process from the magnetosphere to the ground. It is partly due to the facts that a limited number of satellites are not capable of tracing the propagation of disturbances in a highly refractive system such as the magnetosphere, and ground- based measurements such as magnetometers are allowed to see only secondary signals which are modified and reproduced by the ionosphere. Recently, Lee and Kim [Lee and Kim, 2000] studied the propagation properties in a time-dependent manner. They reported that the leading edge of impulses undergoes strong refraction because of inhomogeneous Alfven speed in a dipolar magnetosphere. The travel time between the equatorial magnetopause and the polar ionosphere was found to be shorter than that between the magnetopause and the deep plasmasphere [Lee and Kim, 2000]. We concentrated our study on the fast forward shocks. In our exploration, we used events from the IP shock database compiled by Kasper (Kasper Interplanetary shock database http://space.mit.edu/home/jck/shockdb/shockdb.htm) during last three months of 2001 year. The fast MHD wave fronts were observed inside the magnetosphere by GOES-10, Cluster, and Polar. 133 WDS'08 Proceedings of Contributed Papers, Part II, 133–137, 2008. ISBN 978-80-7378-066-1 © MATFYZPRESS