Current Density and Wave Polarization Observed in Density Holes Upstream of Earth's Bow Shock

Abstract

Previous articles [1,2] have shown density holes are regions of density depletions below the solar wind level with scale length of an ion gyroradius. Density holes have steepened edges seen in both particles and fields. Here we present first observations of currents and wave polarizations associated with density holes. We show an example of current density determined from four point Cluster observations that has a value ~150 nA m^-2. The waves are elliptically polarized and rotating in the sense of ions (left hand) in the plasma frame. The significance of these observations are still being studied. The waves appear to grow and steepen as the density holes are convected with the solar wind toward the Earth. The transient nature of density holes suggests that the temporal features could represent the different stages of nonlinear evolutionary processes that produce a shock-like structure.