Ion Holes Observed in the Upstream Solar Wind

We report novel observations by the Cluster and Double Star satellites of magnetic and ion hole structures upstream of the bow shock that are much smaller and more frequent than those that have previously been reported. Their durations are typically 20-30 s but can be as brief as 8 s, with densities more than an order of magnitude below that of the solar wind. The ion holes are accompanied by similarly shaped magnetic holes. The electric field at the edges is bipolar and magnetic field either compression or rotation. Multi-spacecraft timing analysis shows these holes typically have sizes about an ion gyroradius but are expanding. An intriguing possibility is that these ion holes are the long-sought nonlinear solitary Alfvenic ion density structures in the solar wind predicted theoretically and modeled using hybrid simulations.