Open AccessGeomagnetism and Schmidt quasi‐normalization
Author(s) -
Winch D. E.,
Ivers D. J.,
Turner J. P. R.,
Stening R. J.
Publication year - 2005
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.2004.02472.x
Subject(s) - spherical harmonics , vector spherical harmonics , zonal spherical harmonics , spin weighted spherical harmonics , mathematical analysis , normalization (sociology) , orthonormal basis , spherical coordinate system , solid harmonics , spherical cap , mathematics , tensor operator , spherical mean , vector potential , scalar potential , spherical shell , physics , harmonics , magnetic field , classical mechanics , geometry , quantum mechanics , shell (structure) , sociology , anthropology , materials science , voltage , composite material
SUMMARY Spherical harmonic analysis of the main magnetic field of the Earth and its daily variations is the numerical determination of coefficients of solid spherical harmonics in the mathematical expressions used for the magnetic scalar potential of fields of internal and external origin. The coefficients are determined from vector components of the field and their purpose is to represent the vector field, not to reconstruct the magnetic scalar potential. An alternative interpretation of the spherical harmonic analysis is presented: namely the determination of the coefficients of a series representation of the magnetic vector field on a spherical surface in orthonormal real vector spherical harmonics, which correspond to the internal and external fields, and an additional non‐potential toroidal field. The numerical values of the coefficients of an orthonormal vector spherical harmonic series have a direct physical significance, which is not obscured by some arbitrary normalization of the vector spherical harmonics. Therefore, we propose a Schmidt vector normalization to be used in conjunction with the Schmidt quasi‐normalization of associated Legendre functions. A property of orthonormalized functions is that the standard deviations of the coefficients determined by the method of least squares from ideal data, which are uniformly accurate and uniformly globally distributed, are constant for all coefficients. The real vector spherical harmonic analysis of the geomagnetic field is extended to a spherical shell and conditions that restrict the radial dependence of the vector spherical harmonic coefficients are examined. In particular, two hypotheses for the current systems deriving from the non‐potential toroidal component of the magnetic field over the surface of a sphere are presented, namely, Earth–air currents and field‐aligned currents.