Foreshock Density Holes in the Context of Known Upstream Plasma Structures

We present case examples of foreshock density holes and results from a statistical survey, which provide additional characterizations of these recently-described structures. Specific effort is made to place these objects into context with well-studied foreshock phenomena, such as hot flow anomalies (HFAs) and large-amplitude magnetic pulsations (SLAMS). Density holes are observed during higherthan-average solar wind speeds (∼620 km s), have wellcorrelated density and magnetic field intensities, and anticorrelated density and temperature variations. Like HFAs, these structures occur over a wide range of foreshock geometries, suggesting that this is not a determining factor. They are embedded within IMF current sheets, but their crossstructure magnetic shears are considerably lower than for HFAs. When the Cluster spacecraft are widely separated, they are able to measure structure time development, with substantial changes occurring over 10s of seconds, confirming an earlier case study, and possibly indicating short lifetimes as well. We find that density holes can occur in the absence of strong upstream magnetic pulsations and/or density enhancements, which rules out a ‘wake effect’ as the sole explanation for their formation. Most important is the observation that the observed solar wind motional electric fields tend to have components pointing away from the embedding IMF current sheets. Density holes have no connection with magnetic holes and foreshock cavities, and appear not to be early-stage or weakly-formed HFAs.