Three‐point magnetic field observations of substorms in the inner magnetotail

In 1984–1985 the GOES 5 and GOES 6 spacecraft monitored the Earth's magnetic field at two geosynchronous locations ∼30° apart while the eccentric orbiting AMPTE/CCE spacecraft made equatorial measurements out to 8.8 R E . After intercomparing GOES and CCE data at 6.6 R E on the dayside to reduce uncertainties in the spin axis magnetic field component at GOES, the data sets were studied in the nightside hemisphere. Comparing GOES data with CCE data as the latter spacecraft were passing through the synchronous orbit distance near midnight revealed north‐south (latitudinal) field gradients of the order 40–50 nT/ R E in the range 0.5 to 1.3 R E north of the equatorial plane. Coherent variations at the various spacecraft predominate in the north‐south and radial components of the field and these variations are readily explained as the well‐known plasma sheet reconfigurations associated with substorms. The east‐west field component usually exhibits less coherence between the spacecraft. These differences in the east‐west component at various spacecraft are generally consistent with the current wedge model, but changes in the sign of this component between nearby spacecraft indicate that spacecraft ∼1 R E north of the equatorial plane are frequently immersed in or even north of the wedge currents. Little evidence was found for a high field injection front propagating inward from the tail, but the absence of such evidence may be due to the fact that our dual spacecraft measurements are 0.5–1.3 R E north of the geomagnetic equatorial plane in a strong field region where such injection‐front field increases are unlikely to occur.