Two encounters of the substorm onset region with the Geotail spacecraft

The Earth Camera on board the Polar spacecraft provided high-resolution images for 112 auroral substorms when the Geotail spacecraft was in the nighttime sector of Earth's magnetotail during the period May 1 through September 30, 1996. The purpose of the study was to investigate the dynamical behavior of the plasmas and magnetic fields at the Geotail spacecraft when it was positioned within the central plasma sheet prior to and during the onset of a substorm. In addition, the position of the Geotail spacecraft was required to be within 20 min in local time and several degrees in magnetic latitude of the equatorial position of the magnetic field lines threading the auroral onset region as viewed with the Polar camera and as mapped with a global magnetic field model. For this study these onset mapping locations were required to be earthward of the Geotail spacecraft. Only two substorms, on July 2 and September 4, satisfied these restraints which were imposed in order to observe the dynamics of the central plasma sheet. The radial distances for the Geotail locations for these two substorms were 11.7 and 15.0 R-E, respectively. Before the onsets there was a sustained period of earthward flow of plasma into the extraterrestrial ring current at average speeds of similar to 50 km s(-1). Then, for a period of several minutes before the auroral onset, this plasma flow became stagnant and was accompanied, in part, by fluctuations in components of the magnetic field. At substorm onset a tailward high-speed flow of plasmas at 200-300 km s(-1) began which subsequently subsided within several minutes. The magnetic fields in this tailward directed pulse of plasma flows exhibited a substantial southward component which is accounted for by magnetic merging of field lines at locations inside of the Geotail spacecraft orbit and thus in the ring current or earthward edge of the plasma sheet. Overall, these fortuitous observations support the merging of magnetic field lines at substorm onset in the plasmas with high-energy densities in the ring current but within a very restricted range of local time, several tens of minutes or less.