HYBRID STATE OF THE TAIL MAGNETIC-CONFIGURATION DURING STEADY CONVECTION EVENTS

Previous observations have shown that during periods of steady magneto spheric convection (SMC) a large amount of magnetic flux crosses the p lasma sheet (corresponding to similar to 10 degrees wide auroral oval at the nightside) and that the magnetic configuration in the midtail i s relaxed (the current sheet is thick and contains enhanced B-Z). Thes e signatures are typical for the substorm recovery phase. Using near-g eostationary magnetic field data, magnetic field modeling, and a novel diagnostic technique (isotropic boundary algorithm), we show that in the near-Earth tail the magnetic configuration is very stretched durin g the SMC events. This stretching is caused by an intense, thin westwa rd current. Because of the strongly depressed B-Z, there is a large ra dial gradient in the near-tail magnetic field. These signatures have b een previously associated only with the substorm growth phase. Our res ults indicate that during the SMC periods the magnetic configuration i s very peculiar, with co-existing thin near-Earth current sheet and th ick midtail plasma sheet. The deep local minimum of the equatorial B-Z that develops at R similar to 12 R(E) is consistent with steady, adia batic, Earthward convection in the midtail. These results impose const raints on the existing substorm theories, and call for an explanation of how such a stressed configuration can persist for such along time w ithout tail current disruptions that occur at the end of a substorm gr owth phase.