Modelling Mars' Magnetic Anomalies

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1517 Magnetic Anomaly Modeling, 1550 Spatial Variations Attributed To Seafloor Spreading (3005), 5440 Magnetic Fields And Magnetism

Scientific paper

An internal potential function was derived up to n=90 to improve upon an ongoing study of the crustal structure of the Martian magnetic field. The data included MGS magnetic observations below 200 km altitude taken during the two aerobraking phases (AB1 and AB2), the Science Phase Orbits (SPO), as well as higher altitude (370--440 km) data taken on the nightside during the Mapping Phase Orbits (MPO). The merger of these four sets of data provided total global coverage, and the entire data set was reduced to equal area density based on a 5 degree by 5 degree block at the equator. A spherical harmonic series was developed using techniques similar to those used for Earth field analysis, utilizing all three components of observed field. The resulting model yields an internal dipole field of 2.0 nT and a corresponding magnetic dipole moment of 1.0x 108 A--m2, supporting the experimenter's conclusions of the absence of an internal source. The energy density spectrum of the harmonics is seen to increase with increasing n with a small dip at n=6 and flattening out to approximately 0.5 J/km3 thereafter. Terms below n=6 are an order of magnitude below this figure. This flat, crustal component of the spectrum is two orders of magnitude higher than the same component for Earth, and terms below add little to the spectrum. Contour maps of the components drawn for 200 km altitude show all the major anomalies currently under investigation including two medium anomalies in the northern polar region, two in the northern plains, and the large source centered at 60° S and 180° longitude. Comparisons were made at 200 km with a model derived using equivalent source technique produced by M. Purucker. Differences were found that ranged up to half the value of the strength of the anomalies themselves, though the patterns were similar. The discrepancy between these two techniques suggests that strict criteria need be established by researchers in selecting and reducing the low altitude AB data, before a final crustal magnetization model may be developed with any accuracy. Other workers use only the vertical component data in modeling because the horizontal are said to be more affected by external field contributions. However, the residual misfit distributions show that the vertical component data have only a 24% smaller Gaussian error than the horizontal for the AB data, and at mapping altitudes the difference is negligible. The dayside, low--altitude, AB data has the largest scatter, amounting to 10--14 nT whereas the MPO and nighttime AB ranged 5--7 nT.

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