The Traveling Magnetic Separatix in the Course of Substorms

This study has been carried out using the data for three selected substorms from SO to 120 ground-based magnetometers in the Northern Hemisphere, Phi >40 degrees Using the magnetogram inversion technique, MIT-2. these data were employed in calculating maps of field-aligned current density spatial distribution in the coordinates of latitude-time. The polar cap boundary was then determined for each substorm at 5-minute intervals, as high-latitude boundary of Iijima and Potemra's FAC Region 1. For the load phase of substorms, two types of a gradual extension of the polar cap area was observed, An extension of the Ist (2nd) type begins on the dayside (nightside) of the polar cap. A simple quantitative interpretation is given to these two conspicuous types of p.c, boundary motion. which takes into account for the first time the "old" and "new"' polar caps and reconnection of the old and new open magnetic fluxes.