Figure 17 shows the smoothed monthly geomagnetic index aa as a function of time along with the sunspot number for comparison. The minima in geomagnetic activity tend to occur just after those for the sunspot number and the geomagnetic activity tends to remain high during the declining phase of each cycle. This late cycle geomagnetic activity is attributed to the effects of high-speed solar wind streams from low-latitude coronal holes (cf. Legrand and Simon, 1985). Figure 17 also shows the presence of multi-cycle trends in geomagnetic activity that may be related to changes in the Sun’s magnetic field (Lockwood et al., 1999).
Feynman (1982) decomposed geomagnetic variability into two components – one proportional to and in phase with the sunspot cycle (the R, or Relative sunspot number component) and another out of phase with the sunspot cycle (the I, or Interplanetary component). Figure 18 shows the relationship between geomagnetic activity and sunspot number. As the sunspot number increases there is an increasing baseline level of geomagnetic activity. Feynman’s R component is determined by finding this baseline level of geomagnetic activity by fitting a line proportional to Sunspot Number. The I component is then the remaining geomagnetic activity. These two components are plotted separately in Figure 19.
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