The high-altitude cusps: HEOS 2

Magnetic field data from the HEOS 2 spacecraft are presented for magnetopau se crossings during the summers of 1972-1974. The orbit of HEOS 2 at this t ime was optimal for studying the high-altitude cusp. Magnetopause crossings were categorized as being either tailward or on the dayside of the cusp, o r through the cusp itself. The tailward crossings form a relatively distinc t set, but the dayside and cusp crossings are mixed, indicating either cons iderable motion or distension of the cusp. The cusp crossings extend from 5 .5 to 7.5 R-E in the X-GSM direction and from 4 to 7 R-E in the Z(GSM) dire ction (30% of all crossings). A more detailed survey of five crossings repr esenting each category is presented. Most of these illustrate that the cusp is a very dynamic region, with magnetic field fluctuations occurring on sh ort timescales. Although useful for the above classification, in general th ere is little agreement with the current generation of magnetospheric field models. The gross character and location relative to the model field geome try, however, can be identified, given the prevailing magnetospheric activi ty.