ETS-VI MAGNETIC-FIELD OBSERVATIONS OF THE NEAR-EARTH MAGNETOTAIL DURING SUBSTORMS

The magnetic field experiment on board ETS-VI (the Engineering Test Sa tellite-VI) and initial results from the experiment are presented. ETS -VI was launched on August 28, 1994 and has been placed in a near equa torial orbit with a perigee of 2.3Re, an apogee of 7.1Re, an inclinati on of 13.4 degrees, and an orbital period of 14.4 hours. The spacecraf t is three-axis stabilized. The magnetic field experiment on board the satellite consists of a triaxial fluxgate magnetometer. The magnetome ter has a sampling interval of 3 s, and except near perigee, it is ope rated in the mode having a +/-256 nT dynamic range and a 0.125 nT reso lution. We have examined the magnetometer data obtained at radial dist ances from 5.0 to 7.1Re, magnetic latitudes from -10 degrees to 25 deg rees, and local times from 14 MLT through midnight to 04 MLT, to study magnetic field variations associated with substorms. Substorm-associa ted 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 config uration during the growth phase. The magnetic field becomes highly tai llike and its intensity increases at high latitudes (>10 degrees), whi le the field intensity decreases at lower latitudes (<10 degrees). 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.