Magnetic disturbances in the vicinity of synchronous orbit and the substorm current wedge: A case study

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Abstract
Some features of Pi 2 pulsations and magnetic bays observed on the ground using the mid‐latitude Air Force Geophysics Laboratory (AFGL) Magnetometer Network previously have been shown to agree with ideas concerning the development of a substorm current wedge during isolated substorm onsets. In particular, the H and D component magnetic bay perturbations and the azimuths of the horizontal component of the Pi 2 polarization ellipse observed by the AFGL Network have been used to locate the approximate center and azimuthal extent of the substorm current wedge. In this paper we apply this technique to a well‐defined substorm to locate the substorm current wedge position relative to the GOES 2 and 3 synchronous satellites at L ≃ 6.6 and the nearby SCATHA satellite at L ≃ 8.1 to explain the magnetic features observed by those satellites. We find that the two satellites within the current wedge, at a position just after local midnight, simultaneously observe a dipolarization of the background field configuration beginning at the time of the substorm onset; however, the impulsive, irregular oscillations frequently observed at substorm onset are only observed by one of the satellites. The other satellite within the wedge observes a similar disturbance after a 3‐minute delay which may be attributed to an intensification or movement of currents in the downward current portion of the substorm current wedge. The satellite west of and outside the current wedge observes no significant change in the magnetic field during the event. These observations illustrate the difficulties of interpreting single‐satellite measurements of substorm effects. If magnetic data from only the satellite west of the wedge had been available, the substorm would have gone undetected. Furthermore, an incorrect substorm onset time could have been deduced from one of the satellites within the wedge.