The time sequence of the magnetic field strength, BT, in the magnetotail lobe at a distance of about 15 and beyond during substorms is well known. In the tail lobe over a wide range of distances from the Earth, BT increases during the growth phase or loading phase, then recovers toward the pre-growth phase value during the expansion phase or unloading phase [e.g., Fairfield and Ness, 1970; Camidge and Rostoker, 1970; Russell and McPherron, 1973; Nishida and Nagayama, 1975]. On the other hand, the time sequence of BT in the polar magnetosphere, the magnetic field lines of which are connected to those in the tail, has not been examined until now due to the unavailability of data in this region.
The ISTP spacecraft POLAR with its long dwell-time in the high altitude polar magnetosphere enables for the first time a detailed study of the time sequence of the magnetic field strength during substorms. In this paper we examine the influence of substorm on the polar magnetosphere and present initial results, observed on March 28, 1996.