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 dura...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±10.4s 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 150nA·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.展开更多
基金The research at UC Berkeley is performed under the auspices of a NASA Grant No.NNG04GF23GCluster is a joint project of ESA and NASA and Double Star a joint project of ESA and the Chinese Space Agencysupported by the WCU program through NRF funded by MEST of Korea(R31-10016)
文摘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±10.4s 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 150nA·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.
