Ordering the Earth's magnetic field by geocentric magnetospheric equatorial coordinates: Lessons from HEOS

HEOS high-resolution magnetic field measurements have been recovered and ar e used here to complete and regenerate an early study of the Earth's cusp f ield geometry, first made by Hedgecock [1975]. We make use of a special coo rdinate transformation, introduced by Hedgecock and Thomas [1975], to merge data from cusp crossings corresponding to different dipole tilt angles. Th e coordinates correspond to a geocentric magnetospheric equatorial (GME) sy stem which was defined originally to apply both to position and the magneti c field. The transformation can be extended to incorporate other magnetotai l models, for example, and these and other modifications are being investig ated. Low-resolution summary data are merged over the whole mission from bo th HEOS -1 and -2 to provide global coverage of the magnetospheric field. T his is found to be well ordered in GME coordinates, which remove to a large degree the effect of the dipole orientation on field geometry. Use of the Tsyganenko [1989] field model las well as recent magnetopause models), whic h was not available for the 1975 study, can assist in such an investigation and provides a baseline test of the ordering effect of GME.