Observations of magnetic field dipolarization during auroral substorm onset

The dynamical behavior of plasmas and magnetic fields in the vicinity of the equatorial crossing of magnetic: field lines threading the onset auroral are is examined for two substorms on November 26, 1997. The locations of the initial brightenings of the auroral arcs were determined with the cameras for visible and far-ultraviolet wavelengths on board the Polar spacecraft. The equatorial positions of the field lines were in the range of radial distances of 8-12 R-E as computed with models of Earth's global magnetic field. The radial distance of the Geotail spacecraft was 14 R-E at a position in the premidnight sector that was 2 R-E below the current sheet. This spacecraft was embedded in a low-beta plasma that was located adjacent to the central hot plasma sheet. For the first substorm, with onset at 1310 UT, no substantial effect was observed in the plasmas and magnetic fields, although the Geotail spacecraft was located only about 2 hours in magnetic local time from the field lines threading the onset auroral are. For the second substorm onset, at 1354 UT, the spacecraft was positioned within tens of minutes in local time of the position of the magnetic field lines threading the onset auroral are. This fortuitous spacecraft position in the relatively quiescent plasma and magnetic fields adjacent to the central plasma sheet and within several Earth radii of the position of the onset mechanism allowed determination of the beginning time of the dipolarization of the magnetic fields. This time was simultaneous with the onset brightening of the auroral are within the approximately 1-min time resolution of the auroral images. The simultaneity of the initial brightening of the auroral are and of the initiation of the dipolarization of the magnetic field, presumably due to diversion of current from the equatorial current sheet to the ionosphere, provides an important guideline for global dynamical MHD models of Earth's magnetosphere.