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

HEOS high-resolution magnetic field measurements have been recovered and are used here to complete and regenerate an early study of the Earth's cusp field geometry, first made by Hedgecock [1975]. We make use of a special coordinate transformation, introduced by Hedgecock and Thomas [1975], to merge data from cusp crossings corresponding to different dipole tilt angles. The coordinates correspond to a geocentric magnetospheric equatorial (GME) system which was defined originally to apply both to position and the magnetic field. The transformation can be extended to incorporate other magnetotail models, for example, and these and other modifications are being investigated. Low-resolution summary data are merged over the whole mission from both HEOS -1 and -2 to provide global coverage of the magnetospheric field. This 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), which was not available for the 1975 study, can assist in such an investigation and provides a baseline test of the ordering effect of GME.