GEOTAIL observations of magnetic reconnection in the near-Earth magnetotail Original Research Article

We have found clear evidence for intensification of the cross-tail current and associated occurrence of magnetic reconnection starting from several minutes before a substorm onset at a distance of ∼17 Re down the tail. Following a gradual increase in the growth phase, the current density was intensified stepwise at ∼5 minutes before the onset, and then gradually increased on the average while fluctuating. It is noted that the intensification of the cross-tail current density was due to enhancement of the dawnward electron velocity. The increasing current density accompanied a gradual increase of the tailward flow velocity and small enhancements of the electron and ion temperatures. A few minutes later, the magnetic Bz field dropped down to very small values, and shortly thereafter (around the substorm onset) the rate of change of the tailward velocity was enhanced in association with negative Bz field, significant increasing of the ion and electron temperatures, and decreasing of the plasma density. These signatures can be well understood in terms of temporal evolution of magnetic reconnections associated with local thinning of the current sheet and intensification of the current density.