Nonlinear Evolution of the Upstream Edge of the Density Holes in the Solar Wind Upstream of the Bow Shock

In this study we present new observations of the nonlinear evolution of the upstream edge of the density hole into a shock-like structure. In 2003 the Cluster constellation was largely elongated in the Sun-Earth direction, with the maximum separation of ~1.5 RE. In this configuration the Cluster have observed the temporal evolution of the upstream edge of the density holes as they passed through the constellation. In the early phase of the evolution the solar wind was compressed at the upstream edge and formed a compressional pulse that resembled a solitary wave. In the spacecraft frame the pulse had a right-handed elliptical polarization. In the late phase the upstream edge steepened with the amplitude growing to δB/B0 > ~2, evolving into a shock-like structure. We also observed behind the steepened edge a downstream- like plasma which grew as the density hole evolved. In the late phase when the downstream-like region had largely developed, gyrating components from the shock-like structure were detected and the ion distributions became more isotropic. This observation will exhibit how the shock evolves from the compressional wave and how the ions and magnetic field dynamically interact in the process.