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Empirical orthogonal function analysis of magnetic observatory data: Further evidence for non‐axisymmetric magnetospheric sources for satellite induction studies
Author(s) -
Balasis Georgios,
Egbert Gary D.
Publication year - 2006
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2006gl025721
Subject(s) - ring current , physics , asymmetry , empirical orthogonal functions , geomagnetic storm , earth's magnetic field , local time , magnetosphere , geophysics , ionosphere , observatory , satellite , current (fluid) , computational physics , geodesy , astrophysics , magnetic field , geology , astronomy , climatology , mathematics , statistics , quantum mechanics , thermodynamics
Although satellite electromagnetic induction studies have usually assumed a symmetric magnetospheric ring current source, there is growing evidence for significant source asymmetry. Here we apply empirical orthogonal function methods to mid‐latitude night‐side hourly mean geomagnetic observatory data to search for evidence of non‐zonal low‐frequency source fields. The dominant spatial mode of variability in residuals, obtained by subtracting symmetric ring current and ionospheric fields of the CM4 comprehensive model, has a substantial Y 2 −1 quadrupole component and is highly correlated with D st . This pattern of temporal variability, which implies enhanced ring current densities in the dusk sector, persists even when peak storm‐time data are omitted. The observed asymmetry agrees with that inferred previously by Balasis et al. (2004), from the local time dependence of biases in satellite induction transfer functions. Temporal correlation of the leading mode with D st , and consistency of its spatial structure with recent empirical ring current models, suggest a magnetospheric origin.