A comprehensive statistical study of foreshock density holes

Recently, Parks et al [2006]1 reported on the occurrence of sub-minute duration, order-of-magnitude density and magnetic field reductions within regions of Earth's foreshock. That paper included results from a preliminary survey of ~145 events seen by Cluster during a small number of foreshock transits. Here we present a first comprehensive survey of foreshock density holes, including events from many distinct spacecraft passes, and a significantly more robust description of the events. Characterizations include the solar wind plasma and interplanetary magnetic field (IMF) conditions in which they arise, flow and IMF shears across the structures, and values for the normal components of the convection electric fields. Where possible, boundary normals and velocities, scale sizes, local geometries and Mach numbers, as well as downstream shock geometries are determined. Comparison will be made with published characterizations of other foreshock structures, including large- amplitude magnetic pulsations, hot flow anomalies and foreshock cavities. 1Parks et al 2006, Parks et al 2007.