Coerte V. Voorhies

An altitude-normalized magnetic map of Mars and its interpretation

Techniques developed for the reduction and analysisofterrestrialsatellitemagneticelddataareusedto better understand the magnetic eld observations made by Mars Global Surveyor. A global distribution of radial (Br) magnetic eld observations and associated uncertainties is invertedfor an equivalentsource magnetization distribution and then used to generate an altitude- normalized map of Br at 200 km. The observations are well-represented by a potential function of crustal origin, consistent with a rema- nent origin for the Martian magnetic features. The correla- tion between the 40546 Br observations andBr calculated from the magnetization solution at observation locations is 0.978. For a magnetization distribution connedto a 50 km layer,calculatedmagnetizationsrangefrom-22to+17A/m. We see correlations with tectonics that were only hinted at in earlier maps. Magnetic features appear to be truncated against Valles Marineris and Ganges Chasma, suggestive of a major change in crustal properties associated with fault- ing.

Ørsted Initial Field Model

Magnetic measurements taken by the Orsted satellite during geomagnetic quiet conditions around Jan-uary 1, 2000 have been used to derive a spherical harmonic model of the Earths magnetic field for epoch 2000.0. The maximum degree and order of the model is 19 for internal, and 2 for external, source fields; however, coefficients above degree 14 may not be robust. Such a detailed model exists for only one previous epoch, 1980. Achieved rms misfit is < 2 nT for the scalar intensity and < 3 nT for one of the vector components perpendicular to the magnetic field. For scientific purposes related to the Orsted mission, this model supercedes IGRF 2000.

Time-varying fluid flow at the top of Earth's core derived from definitive geomagnetic reference field models

The effects of a time-varying fluid velocity have been included in the estimation of frozen-flux core surface flow. This has been done to improve the quality, performance, and accuracy of kinematic analyses of the geomagnetic secular variation. The derived flows consist of a reference velocity plus a steady acceleration and thus evolve linearly with time. The method features simultaneous solution for the initial radial geomagnetic field component at the core-mantle boundary and the subadjacent, time-varying fluid motion. The trade-off between time-averaged spatial complexity of the derived flows and misfit to the weighted Definitive Geomagnetic Reference Field models for 1945-1980 is explored. At fixed flow complexity, simultaneous solution for both initial geomagnetic field and steadily accelerating flow gives substantial reductions in the square-weighted residuals per degree of freedom when compared with previous solutions for either initial field and steady flow or steady flow alone. The apparent timescale for flow change varies nonmonotonically with flow complexity between about 23 and 41 years ; the single maximum value is associated with unit negative slope of the log-log trade-off curve and is used to help identify a preferred solution. This solution has an rms speed of but 7.50 km yr -l and an rms acceleration of 0.183 km yr -2 , or 1.84X10 -13 m s -2 . The latter indicates extremely weak net forces upon the liquid outer core.

Testing recent geomagnetic field models via magnetic flux conservation at the core-mantle boundary

Abstract The 28 candidate main field models submitted for the International Geomagnetic Reference Field Model Revision 1985 reveal that a dramatic increase in the rate of decline of the Earths total pole strength began around 1960. Nevertheless, all but one field model conserves the absolute magnetic flux linking the core-mantle boundary (CMB) fairly well during the 40 year time span 1945–1985. This supports the frozen-flux approximation on global length and decade time scales. Geostrophic fluid motion at the top of a frozen-flux core conserves the total magnetic flux linking either geographic hemisphere of the CMB. This constraint is poorly satisfied by the candidate secular variation (SV) models. Yet other SV models truncated at higher degree, and models based upon temporal interpolation rather than extrapolation, satisfy this constraint very well. These results lead us to make several recommendations for the 1985 and future IGRF revisions.