[1] A potential function using spherical harmonics up to degree and order 90 was derived from a selection of Mars Global Surveyor vector data. These included all three components of those taken below 200 km altitude during the two aerobraking phases (AB1 and AB2), the Science Phase Orbits (SPO), and the higher-altitude (367-435 km) data taken on the nightside during the Mapping Phase Orbits (MPO). The merger of these sets of data provided total global coverage. The technique used was a least squares minimization developed for Earth field analysis whereby the relative weighting of each data source was determined by the width of a Gaussian fit to the residual distribution about the potential function. Also, data selection by area and area weight functions were used to improve normalization. The residual misfit distributions for the vertical component for different data sources are (nT) AB1, 6.5; AB2 (in shadow), 6.7; AB2 (sunlit), 10.3; SPO, 6.4; and MPO, 5.9. The horizontal component misfits are about the same for MPO and AB2 in shadow, but for data taken in sunlight the scatter of horizontal component residuals increases by 50% for AB1, a factor of two for SPO, and 30% for AB2. The energy density spectrum evaluated at 3535 km radius (the mean altitude of the AB and SPO data) decreased from a high of 0.2 J/km 3 near n = 20 to an order of magnitude less at n = 90. Most of the power in the spectrum lies between n = 15 and n = 40. The dipole moment was only 8 Â 10 16 A Á m 2 , which is likely close to the noise of the coefficients. This spectrum is 40 times greater than that of Earth at the scale sizes represented by values of n from 20 to 40. Comparisons with other published maps and models for Mars show general agreement with the field representations at MPO altitudes, but disagreements up to several hundred nT in components calculated for areodetic altitudes below 200 km.

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[1] A study was done to determine the difficulty of obtaining a field model for 1995 using magnetic observations taken since that epoch. Ørsted vector data taken from 1999 to 2001 in conjunction with scalar data from all surface ship surveys and secular variation data from magnetic observatories taken since 1995 were utilized. Spherical harmonic models were constructed for the interval from 1995 through early 2001 using selected, magnetically quiet, nighttime intervals. Corrections were made for Dst variations assuming a constant, i.e., ratio (0.28). The data were corrected for magnetic anomalies using Magsat coefficients (m102389) above n = 21. In order to compute secular change data, some observatory hourly values were reduced to remove the annual variations, which ranged up to 40 nT at a few stations. Coefficients were adjusted up to n = 21 in spatial terms, n = 13 in linear secular variation terms, and n = 12 in parabolic secular variation terms. There was also a 30 nT, n = 1 external term. Component maps of this model (f052101) truncated to n = 10 were compared with the IGRF2000 and generally found to be within 20 nT at the surface except for a 110 nT difference in the northern polar region. The model agreed with the OIFM at satellite altitude to within a few nanoteslas except for a few tens of nanoteslas at the poles. Model differences at 1995 from the IGRF1995 peaked over 800 nT in the region west of South America. However, statistical estimates in this region and epoch show model errors up to several hundred nanoteslas. Use of a linear model or one truncated to lower degree is seen to reduce these differences and maximum errors at 1995, but at the expense of less accuracy elsewhere. Besides the large differences from the 1995 IGRF it was also noted that there are a number of areas where the field changes require parabolic coefficients, even over this short a time span.