Larmor radius size density holes discovered in the solar wind upstream of Earth’s bow shock

Abstract

The Cluster and Double Star satellites recently observed plasma density holes upstream of Earth’s collisionless bow shock to apogee distances of 19 and 13 earth radii, respectively. A survey of 147 isolated density holes using 4 s time resolution data shows they have a mean duration of 17.9±10.4 s, but holes as short as 4 s are observed. The average fractional density depletion (δn/n) inside the holes is 0.68±0.14. The upstream edge of density holes can have enhanced densities that are five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock. Multispacecraft analyses show the density holes move with the solar wind, can have an ion gyroradius scale, and could be expanding. A small normal electric field points outward. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The density holes are only observed with upstream particles, suggesting that backstreaming particles interacting with the solar wind are important.