Inherent vector discrepancies in geomagnetic main field models based on scalarF

Abstract

We have investigated the spherical harmonic analysis (SHA) of scalar intensity F. The method used was the analysis of simulated data, derived by adding pseudorandom noise to surface values (with equiangular spacing) synthesized from existing spherical harmonic coefficients (SHC). An advantage of using simulated data is that this makes possible the computation of vector as well as scalar residuals. In general, similar results were obtained regardless of whether the number of input SHC used in the synthesis of simulated data was less than, equal to, or greater than the number of output SHC. The main result was that the largest vertical intensity residuals occurred in the equatorial region, the foci following the magnetic dip equator rather than the geographic equator. On account of the equiangular spacing, sine colatitude (sinθ ) weighting was usually used in the SHA. Analyses with other weights at θ = 85° and 95° resulted in no improvement. Substitution of vector for scalar data at these colatitudes gave a much better result. This offers some hope that an improved vector model might be obtained from a real data set composed chiefly of F measurements if it incorporated additional vector measurements in selected areas. Complete vector measurements would of course be still better.

Error Analysis of Total Field Models Derived from POGS Data

“…It has already been shown that the Backus effect can be significantly reduced when a few vector data from ground-based observatories are included in the field model derivation (Hurwitz and Knapp, 1974;Stern and Bredekamp, 1975). This effect is more pronounced when these observatories are located near the equator (Barraclough and Nevitt, 1976;Lowes and Martin;1987).…”

Magnetic field modelling from scalar-only data: Resolving the Backus effect with the equatorial electrojet

“…Considerable effort has been devoted to finding a method by which satellite intensity data could be used to produce high-quality field models. It was found that including vector component data from magnetic observatories, especially close to the equator, would significantly reduce the effect of the non-uniqueness (Hurwitz and Knapp, 1974;Barraclough and Nevitt, 1976;Gubbins and Bloxham, 1985;Lowes and Martin, 1987;Ultré-Guérard and Achache, 1997). This observation was explained by Khokhlov et al (1997), who demonstrated that, in principal, the Backus effect could be eliminated with knowledge of the position of the geomagnetic dip equator, the points on the Earth's surface at which the vertical component of the field is zero.…”

Main field and secular variation candidate models for the 12th IGRF generation after 10 months of Swarm measurements

“…This occurs mostly in the equatorial region, and it results in large differences in the vertical component. This effect was discovered and described when no spacecraft vector magnetic field measurements were available (e.g., Backus 1970;Hurwitz and Knapp 1974;Lowes 1975;Stern and Bredekamp 1975). Different strategies have been proposed to alleviate it.…”

“…Different strategies have been proposed to alleviate it. Hurwitz and Knapp (1974) were probably the first to include vector data in the equatorial region, to better constrain the position of the magnetic equator and resolve the sectoral harmonics. These additional data can be provided by the magnetic observatories, which have however a poor geographic distribution.…”