Secular Variations of the Planetary‐Scale Geomagnetic Anomalies

Abstract

The characteristics of a planetary-scale geomagnetic anomaly are described by using "unsigned magnetic flux" through the anomaly region. The eighth generation of the international geomagnetic reference filed (IGRF) is analyzed to study the secular variations of 5 planetary-scale geomagnetic anomalies during the 1900-2000 period. The results show that the magnetic fluxes through the southern Atlantic anomaly (SAT), Australian anomaly (AUS), and African anomaly (AF) have increased by more than 200 MWb, the increase of the flux through the Eurasian anomaly (EA) is smaller (157 MWb), while the flux through the North America anomaly (NAM) has decreased by 50 MWb. On the other hand, the variations of the anomaly areas are within 10%. The westward drift at the core-mantle boundary (CMB) is less than 0.1 • /a, being much slower than that at the Earth's surface (0.2 • /a). This contrast may be attributed to the different westward drift velocities of the various harmonics in the geomagnetic field: the westward drift at the Earth's surface is controlled by the dominant low-degree harmonics, while the westward drift at the CMB largely depends on high-degree harmonics. Consequently, it should be careful to take the westward drift velocity at the Earth's surface as the characteristic velocity of the fluid flow in the outer core.

Seasonal Variations of Sq Current System in Different Longitudinal Sectors and Solar Activities

“…Spherical harmonic analysis was adopted (Chapman & Bartels, 1940; Liu et al., 2021; Stening & Winch, 2013; Takeda, 2002; Xu, 2003; Xu & Li, 1994; Yamazaki, Yumoto, Uozumi, et al., 2011b; Zhao et al., 2014) to model the Sq internal and external seasonal current systems in each chain. The details of SHA method could be found in previous literature (e.g., Liu et al., 2021; Matsushita & Maeda, 1965; Yamazaki & Maute, 2017).…”

“…Stening and Winch (2013) and Yamazaki and Maute (2017) used ground data and simulation, they found that the longitude variations of Sq currents show that the current intensity in the North-America sector is stronger in all seasons. The geographic latitude of the Sq current focus also changes with longitude (Matzka et al, 2009;Sabaka et al, 2002Sabaka et al, , 2015Xu & Li, 1994;Yamazaki & Maute, 2017;Çelik, 2013).…”

Seasonal Variation Characteristics of Geomagnetic Sq External and Internal Equivalent Current Systems in East‐Asia and Oceania Regions

“…r = R 0 is assumed, because (Matsushita & Maeda, 1965). The expansion series is truncated at M = 8, N = 60 (Liu et al., 2019; Xu & Li, 1994; Zhao et al., 2014) based on the two considerations: (1) when fitting S q variations by Fast Fourier transform (FFT) analysis, the first eight order harmonic waves can fit the observation with less than 1% error, and (2) when using Legendre function, the average inflection point of fitting error appears at the 60th step. The cubic spline function is applied to interpolate the triangle coefficients into 1° grid at latitudinal direction after FFT analysis.…”

“…S q equivalent current system can be determined by using spherical harmonic analysis (Chapman & Bartels, 1940; Liu et al., 2019; Stening & Winch, 2013; Takeda, 2002; Xu, 2003; Xu & Li, 1994; Yamazaki, Yumoto, Cardinal, et al., 2011; Zhao et al., 2014). In this study, SHA was utilized to deduce both internal and external S q equivalent current systems.…”

“…r = R 0 is assumed, because  0 0 « r R R (Matsushita & Maeda, 1965). The expansion series is truncated at M = 8, N = 60 (Liu et al, 2019;Xu & Li, 1994;Zhao et al, 2014) based on the two considerations:…”