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Backus Effect on a Perpendicular Errors in Harmonic Models of Real vs. Synthetic DataMeasurements of geomagnetic scalar intensity on a thin spherical shell alone are not enough to separate internal from external source fields; moreover, such scalar data are not enough for accurate modeling of the vector field from internal sources because of unmodeled fields and small data errors. Spherical harmonic models of the geomagnetic potential fitted to scalar data alone therefore suffer from well-understood Backus effect and perpendicular errors. Curiously, errors in some models of simulated 'data' are very much less than those in models of real data. We analyze select Magsat vector and scalar measurements separately to illustrate Backus effect and perpendicular errors in models of real scalar data. By using a model to synthesize 'data' at the observation points, and by adding various types of 'noise', we illustrate such errors in models of synthetic 'data'. Perpendicular errors prove quite sensitive to the maximum degree in the spherical harmonic expansion of the potential field model fitted to the scalar data. Small errors in models of synthetic 'data' are found to be an artifact of matched truncation levels. For example, consider scalar synthetic 'data' computed from a degree 14 model. A degree 14 model fitted to such synthetic 'data' yields negligible error, but amplifies 4 nT (rmss) added noise into a 60 nT error (rmss); however, a degree 12 model fitted to the noisy 'data' suffers a 492 nT error (rmms through degree 12). Geomagnetic measurements remain unaware of model truncation, so the small errors indicated by some simulations cannot be realized in practice. Errors in models fitted to scalar data alone approach 1000 nT (rmss) and several thousand nT (maximum).
Document ID
19990092490
Acquisition Source
Goddard Space Flight Center
Document Type
Conference Paper
Authors
Voorhies, C. V.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Santana, J.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Sabaka, T.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Date Acquired
August 19, 2013
Publication Date
July 19, 1999
Subject Category
Geophysics
Meeting Information
Meeting: International Union of Geodesy and Geophysics General Assembly
Location: Birmingham
Country: United Kingdom
Start Date: July 19, 1999
End Date: July 30, 1999
Sponsors: International Union of Geodesy and Geophysics
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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