Centennial variations in sunspot number, open solar flux, and streamer belt width: 2. Comparison with the geomagnetic data

We investigate the relationship between interdiurnal variation geomagnetic activity indices, IDV and IDV(1d), corrected sunspot number, RC , and the group sunspot number RG . RC uses corrections for both the “Waldmeier discontinuity,” as derived in Paper 1, and the “Wolf discontinuity” revealed by Leussu et al. (2013). We show that the simple correlation of the geomagnetic indices with RC n or RG n masks a considerable solar cycle variation. Using IDV(1d) or IDV to predict or evaluate the sunspot numbers, the errors are almost halved by allowing for the fact that the relationship varies over the solar cycle. The results indicate that differences between RC and RG have a variety of causes and are highly unlikely to be attributable to errors in either RG alone, as has recently been assumed. Because it is not known if RC or RG is a better predictor of open flux emergence before 1874, a simple sunspot number composite is suggested which, like RG , enables modeling of the open solar flux for 1610 onward in Paper 3 but maintains the characteristics of RC .