Interplanetary magnetic field By effects on the magnetic field at synchronous orbit

GOES 3 magnetic field data are analyzed to reveal interplanetary magnetic field (IMF) By effects on the magnetic field configuration near the magnetic equatorial plane at synchronous orbit. The magnetic equatorial plane on the nightside moves upward (northward) in the evening sector and downward (southward) in the morning sector for the positive IMF By. The magnetic equatorial plane moves in the opposite sense for the negative IMF By. A perturbation appears in the azimuthal component of the magnetic field in the midnight sector (2100-0300 LT), and its magnitude is a fraction 0.3 of the IMF βgamma near midnight. These changes are attributable to the IMF By-induced asymmetry of the reconnection. Observations of substorm-associated field-aligned current effects depend strongly on the geometry of synchronous spacecraft relative to these currents so that the IMF By-induced distortion of the magnetic equatorial plane produces apparent IMF By effects on D observations during substorms