Geomagnetic models and the solar cycle effect

Abstract

An examination of first differences in annual means of the northward component at widely separated observatories over an 80‐year period shows many large spikes. These spikes occur simultaneously at many observatories and coincide with rapid changes in sunspot numbers, which shows that they are real and that they come from external sources. A smoothed time derivative curve of the same component made by plotting, for each year, the slope of a straight line determined by least squares from five consecutive years still shows a very strong solar cycle effect. It is important to know how this solar cycle effect modifies the spherical harmonic coefficients of geomagnetic models. A simple example illustrates that a zonal external field makes no contribution to internal spherical harmonic coefficients when a least squares solution is made using a uniform distribution of all component data. This effect is an example of the general theorem that any harmonic field of external origin is orthogonal to any harmonic field of internal origin. Totally different results are obtained if a single component or the total field intensity is used in the least squares solution.

Secular variation from monthly means from Apia and Amberley magnetic observatories

“…The rapid change that we have tentatively associated with the 1970 jerk appears effectively instantaneously at each conservatory, despite our improved time resolution, but there is a lag of some 2. yr between its appearance at Apia and Amberley. The sudden onset in itself points to an external cause because very sudden changes would be screened by the electrical conductivity of the lower mantle, and the slight correlation with sunspot number supports Alldredge's (1982) belief that the phenomenon has an external cause. On the other hand the time difference suggest either that the jerk was not a simultaneous event all over the world, or perhaps that the effects we have studied here are quite separate.…”