Magnetic field signatures of substorms on high-latitude field lines in the nighttime magnetosphere.

Two types of magnetic field changes are repeatedly observed in the high-latitude nightside magnetosphere in association with magnetic substorms. One type of field change occurs on field lines associated with the high-latitude part of the auroral oval and is characterized by a sudden decrease in the field strength accompanied by an abrupt perturbation in the field declination angle. These changes are attributed to field-aligned sheet currents flowing on the high-latitude boundary of an expanding plasma sheet following substorms. A second type of field change observed on polar cap field lines is a decrease in field inclination during substorms. This type of change is regarded as a further manifestation of the changing field configuration during substorms and can be described in terms of azimuthal currents.