ETS-VI Magnetic Field Observations of the Near-Earth Magnetotail during Substorms

The magnetic field experiment on board ETS-VI (the Engineering Test Satellite-VI) and initial results from the experiment are presented. ETS-VI was launched on August 28, 1994 and has been placed in anear equatorial orbit with a perigee of 2.3Re, an apogee of 7.1Re, an inclination of 13.4°, and an orbital period of 14.4 hours. The spacecraft is three-axis stabilized. The magnetic field experiment on board the satellite consists of a triaxial fluxgate magnetometer. The magnetometer has a sampling interval of 3 s, and except near perigee, it is operated in the mode having a ±256 nT dynamic range and a 0.125 nT resolution. We have examined the magnetometer data obtained at radial distances from 5.0 to 7.1Re, magnetic latitudes from-10° to 25°, and local times from 14 MLT through midnight to 04 MLT, to study magnetic field variations associated with substorms. Substorm-associated field variations are easily seen at larger radial distances and at local times later than 19 MLT. Using 92 substorm events that occurred in the 21-01 MLT sector, we have constructed the average field configuration during the growth phase. The magnetic field becomes highly taillike and its intensity increases at high latitudes (> 10°), while the field intensity decreases at lower latitudes (<10°). The field configuration suggests that the inner edge of the growth phase current system is located near the synchronous altitude at the end of the growth phase.