Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmgp21a0031p&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #GP21A-0031
Mathematics
Logic
0925 Magnetic And Electrical Methods, 1527 Paleomagnetism Applied To Geologic Processes, 1535 Reversals (Process, Timescale, Magnetostratigraphy), 5440 Magnetic Fields And Magnetism, 6225 Mars
Scientific paper
Helbig (1963, Zeitschrift für Geophysik) developed the theory for estimating total magnetization vectors from the moments of measured magnetic component data. Helbig's theory has been tested successfully on aeromagnetic data for several terrestrial locations, where both the induced and remanent magnetizations of sources were known (Schmidt and Clark, 1997, Preview; 1998, Exploration Geophysics). Because Mars no longer has an inducing core field, application of Helbig analysis to the vector magnetic field measured by Mars Global Surveyor permits estimation of crustal remanent magnetization directions and strengths, paleomagnetic pole positions, and reversal characteristics of the extinct Martian core-field dynamo. The results provide an independent test of pole positions estimated from forward modeling (Arkani-Hamed, 2001, GRL) and of magnetization models estimated using inversion (Whaler and Purucker, 2003, The Leading Edge). A Helbig analysis of the n=90 Martian magnetic field model (Cain and others, 2003, JGR), evaluated at 150 km altitude, suggests that most of the stronger sources (average magnetization magnitudes > 4 A/m) have paleomagnetic pole positions within 50 degrees of 195E 50N. This region encloses Arkani-Hamed's (2001) estimated pole position of 230E 25N, based on his analysis of ten semi-isolated anomalies. A smaller number of strongly magnetized sources have pole positions that cluster within 40 degrees of 290E 5N. This cluster may represent either a secondary pole position or a preferred transition path during field reversals. Both normal and reversed magnetizations cluster at these pole positions, supporting the existence of a reversing core dynamo during the early history of Mars. When all estimated magnetic sources are included in the analysis, the weaker sources dominate, and the pole positions cluster along 35N and 35S latitudes. The corresponding source locations are concentrated along lines of longitude in areas of low magnetic intensity. These weak magnetic sources are probably due to orbital noise from external fields and do not represent the Martian crust. The vector components of the magnetization estimated from Helbig analysis resemble the preliminary magnetization model of Whaler and Purucker (2003). The strongly lineated magnetic anomalies on Mars, however, pose a difficulty to both magnetization analyses. For lineated anomalies, only the component of the magnetization perpendicular to the strike can be estimated from the data. The missing magnetization components parallel to strike probably contribute to the large scatter seen in the pole positions.
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