The magnetic field and electric current distributions in the magnetotail at the time of onset of a magnetospheric substorm are studied by using the 1989 version of the Tsyganenko magnetic field model. It is assumed that the development of the magnetic configuration in the magnetotail during the growth phase of a substorm stretches the tail until a disruption of the cross-tail current at the time of the substorm onset takes place. The Tsyganenko field model is modified to take into account the development in the magnetic field configuration during the growth phase: the increase of the tail flux and the thinning of the near-Earth plasma sheet. In the model field, both the thinning of the current sheet and the increase in the tail flux stretch field lines tailward, thus making the field more tail-like even in the near-Earth region. If a field-aligned mapping is performed from the ionosphere into the magnetotail, the endpoint at the current sheet using the modified model can vary by more than 10 R(E) from the result obtained with the basic model. The ionospheric footpoint of the tail-like field lines moves several degrees equatorward due to the modifications. The mapping of an auroral arc recorded by all-sky cameras on 15 December 1981 moves earthward by several R(E) during the growth phase of the substorm due to the equatorward motion of the arc. The earthward motion of the mapping can be replaced by stretching of the field to correspond to the assumed development during the growth phase of the arc. To obtain this result, the cross-tail current needs to be enhanced by a factor of 1.3, which still does not produce significant changes in the overall field pattern. The substorm onset is described as a disruption of the cross-tail current, modeled by a current meander in the cross-tail current. The forming current profile compresses field lines toward the current sheet, and plasmoid-like structures form in the tail with typical X-O type geometry in the magnetic field.
- PLASMA SHEET