Observed magnetic substorm signatures at synchronous altitude

Magnetic field data from the geostationary satellites GOES 2 and GOES 3 are examined to study the development of substorm activity in the near‐earth nightside magnetosphere (around a radial distance of 6.6 R E ). Substorm events are those in which a well‐defined single onset is seen at low latitudes on the ground. The field configuration change from more taillike to more dipolelike starts initially in a longitudinally localized region in association with the ground onset, and it develops westward and eastward, even when the simultaneous onset of the low‐latitude positive bay is recorded in a wide longitudinal region on the ground. It is also found that the variation caused by the field‐aligned current starts at the ground onset and reaches a peak at the time of the field change. The present results are consistent with the view that a substorm is associated with a disruption and subsequent conversion of the cross‐tail current to the field‐aligned current connected with the polar ionosphere. It is indicated that the cross‐tail current near synchronous orbit and its disruption are important in producing the field configuration change at synchronous orbit.