A least-squares technique is developed to obtain dipoles, which, when combined with an existing global field model, represent the relatively isloated magnetic anomalies which are residual to the high order (greater than 20) spherical harmonic internal geomagnetic field model. This combination of models (global spherical harmonic and local dipole) forms an efficient and compact representation of the geomagnetic field. The technique involves simulatneously solving for all three components of moment and of position of a dipole. The technique is applied to the mega-anomaly west of Austrailia in a region about Broken Ridge, using data and spherical harmonic models separately constructed from POGO and MAGSAT spacecraft. The resulting equivalent dipoles agree fairly well and may be averaged to a 1 T⋅km3 dipole buried at 60 to 70 km depth near 32¿S, 97¿E oriented horizontally with small (3¿3¿) east declination. |