A Review of Density Holes Upstream of Earth's Bow Shock

Larmor size transient structures with density depletions as large as 99% of ambient solar wind density levels occur commonly upstream of Earth's collisionless bow shock. These “density holes” have a mean duration of 17.9±10.4 s but holes as short as 4 s have been observed. The average fractional density depletion (δn/n) inside the holes is 0.68±0.14. The density of the upstream edge moving in the sunward direction can be enhanced by five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock, and measured local field geometries and Mach number support this view. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. Current densities as large as 150 nA·m^-2 are observed at the leading compressed edge. The waves are elliptically polarized and rotating in the sense of ions (left hand) in the plasma frame. The waves appear to grow and steepen as the density holes convect 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. The density holes are only observed with upstream particles, suggesting that back-streaming particles interacting with the solar wind are important. The significance of these observations is still being investigated.