What Isolated Foreshock Density Holes Tell Us

Short-term (~20~s), order of magnitude depletions in foreshock density and magnetic field intensity have been reported recently. These density holes typically exhibit a greatly reduced solar wind beam, and diffuse-like energetic ions. There is increased wave activity, including enhanced ion cyclotron waves and whistler mode waves in their interiors, along with strong electrostatic waves near density minima. These are often, but not exclusively, observed in the presence of large amplitude ULF waves, or at the interface between quasi-parallel and quasi-perpendicular foreshock regions resulting from abrupt interplanetary magnetic field (IMF) rotations. While they are commonly seen in regions containing large-amplitude magnetic pulsations, the relationship between density holes and those SLAMS has been uncertain. A recently identified statistical sample of density holes occurring without associated SLAMS provides insight into their character and development. Here we present a few representative case studies, including events from 2002, for which short Cluster spacecraft separations permit accurate measurement of boundary normals and speeds, and from 2003 when the inter- spacecraft distances allowed for spatial and temporal variation. The occurrence of events isolated from SLAMS immediately suggests that they cannot be accounted for merely as `wake effects' downstream of flow-diverting magnetic structures. Some cases appear to show structure growth in time, while others indicate collapse. In contrast to original reports for a broader sample of density holes, these isolated cases exhibit moderate-to-strong IMF shear. This may indicate a link to hot flow anomalies that have been examined in detail previously, and which are thought to arise from interactions of solar wind current sheets with the bow shock. Published simulation studies of such mechanisms have not considered small-scale or early development phases, but such results might provide greater insight into the processes leading to density holes.