The smoothed monthly sunspot numbers of the previous 22 complete sunspot cycles are normalized in time domain, and then an eigen mode analysis is carried out to draw the principle factors (or components) in the cycles. The results show that the main characteristics of the solar cycles can be described fairly well by the first 5 eigen modes. The obtained eigen modes are used to predict the declining phase of cycle 23 on the basis of the data prior to its maximum. The prediction indicates that cycle 23 will last for 127 months to December 2006, with the minimum of 6.2.
The method of natural orthogonal components (NOC) is used to analyze the earth抯 main magnetic field IGRF 1900—2000, and the NOC model of the field is established. The first step of the analysis is to calculate eigen modes of the field from the Gauss coefficients of IGRF 1900—2000. Then the magnetic field for each epoch is expanded in a series at the basic function set constructed by the eigen modes, and the intensity coefficients of the eigen modes are calculated. Test of the convergency and stability of the NOC model shows that the model has very short series and much rapid convergency in comparison with the conventional spherical harmonic models of IGRF. Comparison of the eigen modes obtained from different IGRF model groups indicates that the low-degree eigen modes are rather stable, while the high-degree modes show a relatively large variability. The physical meaning of the eigen modes in the NOC model is discussed, and an interesting relationship is found between the spatial structure of the main field and its secular variation.
The main geomagnetic field models of IGRF1900-2000 are used to study the latitude-dependence of the westward drift in the main field. The results show that the latitude-dependence exists in the magnetic components with different wavelengths (m=1-10). The global-average westward drift rate of the component of m=1 is 0.189°/a with the maximum of 0.295°/a at latitudes 40°-45°. The compo-nent of m=2 has an average drift rate of 0.411°/a with the maximum of 1.305°/a at latitude -60°. As for the compo-nents with further shorter wavelengths, the drift is generally restricted in a limited latitude range, and has many smaller drift rates. This latitude-dependence of westward drift can not be explained by rigid rotation of the earth’s core. The results of this note also show that there is a negative disper-sion in the westward drift, namely the components of long wavelengths drift faster than those of short wavelengths. This dispersion feature is not in agreement with Hide’s MHD model. It is likely needed
