Small-scale Bow Shock Motion: Themis Project

The bow shock decelerates and deviates the supersonic solar wind flow around an obstacle and its distance from the obstacle is a function of dimensions of the obstacle, solar wind and interplanetary magnetic field (IMF) parameters. However, the shape and dimensions of the magnetopause that represents the obstacle in the case of the Earth vary if we except upstream parameters also with dipole tilt orientation. The problem is complex because the models describing locations of both boundaries are based on a statistical processing of crossings observed by a single spacecraft. Such crossings locate the boundaries in motion, i.e., in non-equilibrium state and this can be a source of significant errors. We use observations of the dayside low-latitude bow shock crossings and compare them with magnetopause and bow shock empirical models. A fleet of the Themis spacecraft allows us to determine the actual bow shock and magnetopause locations. In present cases, it is important that bow shock observations observed by one Themis spacecraft are complemented with the solar wind data measured just in front of the observed crossings by the other Themis spacecraft. In the paper, we discuss possible sources of a small-scale bow shock motion.