The high‐altitude cusps: HEOS 2

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Abstract
Magnetic field data from the HEOS 2 spacecraft are presented for magnetopause crossings during the summers of 1972–1974. The orbit of HEOS 2 at this time was optimal for studying the high‐altitude cusp. Magnetopause crossings were categorized as being either tailward or on the dayside of the cusp, or through the cusp itself. The tailward crossings form a relatively distinct set, but the dayside and cusp crossings are mixed, indicating either considerable motion or distension of the cusp. The cusp crossings extend from 5.5 to 7.5 RE in the XGSM direction and from 4 to 7 RE in the ZGSM direction (30% of all crossings). A more detailed survey of five crossings representing each category is presented. Most of these illustrate that the cusp is a very dynamic region, with magnetic field fluctuations occurring on short timescales. Although useful for the above classification, in general there is little agreement with the current generation of magnetospheric field models. The gross character and location relative to the model field geometry, however, can be identified, given the prevailing magnetospheric activity.