Magnetic field configuration at geostationary orbit during a substorm

It is shown that at Kp not less than 4 a geostationary satellite near the equatorial plane in the nighttime sector is in a transition region where the geomagnetic field has a character intermediate between the dipole field (predominant in the near magnetosphere) and the magnetotail field. The greatest variations in magnetic field and particle fluxes during a substorm are observed in this region. The behavior of differential disturbance vectors indicates that the inner edge of the current sheet (near which intensification of current density at the preliminary substorm phase and current decay at the substorm commencement occur) can, at Kp not less than 4, be situtated near 6.6 earth radii.