Substorm signatures at synchronous altitude

The signatures of magnetospheric substorms near 6.6 R E are statistically examined by using data obtained on board ATS 6 by magnetic field and energetic particle measurements. It was confirmed that the configuration change toward a taillike field in the dusk‐to‐midnight sector typically begins about one hour before the onset of the expansion phase of a substorm. Configuration changes before the onsets of moderate substorms are characterized by a directional change of magnetic field at magnetic latitude of ∼10°, rather than a change in the field magnitude. During storm periods it was found that the magnetic field orientation occasionally becomes almost parallel to the magnetic equator before expansion phase onsets. In such cases, the field magnitude usually increases above background levels. These facts strongly suggest that a taillilke configuration is caused by an intensification and earthward motion of the cross‐tail current system, often close to 6.6 R E , than the development of diamagnetic ring current. The degree of taillike field distortion is statistically correlated with a subsequent field recovery during the expansion phase. Namely, a large field recovery at ATS 6 and associated large positive bays on the ground tend to follow a pronounced deviation from a quiet field level at ATS 6. The examination of ground magnetograms indicates that the development of S q p or convection bay type current system is associated with the taillike field configuration. These results are discussed in term of a phenomenological model of magnetospheric substorms including the precursory or growth phase.