Observations and simulations of a highly structured plasma sheet during northward IMF

We investigated a highly structured plasma sheet populated by counterstreaming beams observed by the Cluster and Double Star spacecraft on October 10, 2005, during an extended interval of northward interplanetary magnetic field (IMF). We used Wind spacecraft observations as input into our global magnetohydrodynamic (MHD) simulation of the event, and launched ions from the low-latitude boundary layer, determined to be the source of the observed ions, in our large-scale kinetic (LSK) particle tracing calculation that used the time-dependent global fields from the MHD simulation. We found that a highly structured plasma sheet was formed in which the dawn and dusk halves were temporarily decoupled because of the presence of flux ropes in the center of the tail, possibly the result of localized near-Earth reconnection. Our LSK simulation results showed good agreement with observed spectrograms and velocity distribution functions. On the dawn side, the ions launched in our simulation first formed a plasma sheet boundary layer (PSBL). Subsequent interactions of these ions with the current sheet scattered them and formed the central plasma sheet (CPS). On the dusk side, however, the topology of the magnetotail, including the formation of highly stretched field lines in the dusk flank and of a flux rope on open field lines in the center of the tail resulted in a structured plasma sheet, with counterstreaming beams present throughout the plasma sheet, including its center portion. This is very different from the classical picture of the PSBL in which layered counterstreaming beams exist only at the outer edges of the plasma sheet and do not penetrate the CPS.