The effects of a rapid IMF cone-angle change on Earth's magnetopause and boundary fluctuations

One of the most important questions in magnetospheric physics is that of understanding how plasma, momentum, and energy are transferred across the magnetopause due to the interaction of the solar wind with Earth’s magnetic field. The generation of magnetopause waves, often identified as resulting from a Kelvin-Helmholtz instability, has long been considered as playing a significant role in the dynamics of Earth’s boundary layer that separates fast magnetosheath plasma from the relatively stagnant magnetosphere. Although the properties of such waves have been related to certain solar wind parameters, such as the IMF clock angle and the solar wind dynamic pressure, the effect of IMF cone angle has not been studied. In this paper we present Cluster observations of the magnetopause motion and boundary waves in the northern hemisphere on January 12, 2003 during which the IMF cone angle changes from ~90° to ~14°. During this interval changes in the IMF clock angle and in other solar wind parameters are less significant. Cluster and GOES 8 and 10 observations, together with ground magnetometer data, indicate that the magnetopause moves outward following an initial transient inward motion. This motion probably results from the pressure perturbation that is associated with the formation and expansion of the foreshock region upstream of Earth’s bow shock. There exist two superposed magnetopause waves with a frequency of 8 mHz, and 0.6 mHz, respectively, near the arrival of the IMF discontinuity. After the magnetopause moves outward, the surface wave is significantly enhanced and its frequency, amplitude, and propagation direction have all changed. These changes are closely correlated with a change in shear-layer thickness associated with the rarefaction and compression of density and magnetic fields at the boundary that arises due to foreshock dynamics.