Worldwide features of magnetic storms using spherical harmonic analysis of Intermagnet data

SUMMARY The interaction between the solar plasma and the Earth's magnetic field causes a number of current systems to develop in the magnetosphere during a magnetic storm. Compression of the magnetosphere by plasma, magnetopause current, and ring current systems are the main current systems responsible for a typical magnetic storm recorded at mid‐ and low‐latitudes. In an attempt to understand this interaction, spherical harmonic analysis is applied to storm data measured at Intermagnet observatories. As expected, when the variation of the external magnetic field coefficients is examined, the first external field coefficient, γ 0 1 , reveals the phases of the storm more clearly than the other coefficients. Thus, the X ‐component of the Earth's magnetic field was reconstructed for all Intermagnet observatories using the γ 0 1 coefficient together with the equatorial dipole coefficients γ 1 1 , δ 1 1 , and setting the other coefficients to zero. The analysis was repeated using other external coefficients, setting the three dipole coefficients to zero. In this case it is also possible to see the phases of the storm clearly, but no more clearly with the three dipole coefficients. In addition, the relationship between γ 0 1 and Dst , and that between γ 0 1 and the induced field coefficient c 0 1 were analysed, and the regression equations and correlation coefficients calculated for each storm. It is concluded that further studies of these coefficients will give important information about current systems in the magnetosphere, and in coming years such studies will lead to a better understanding of the Earth's magnetic field because of the increase in data quality and the number of observatories contributing to Intermagnet.