The IDV index: Its derivation and use in inferring long‐term variations of the interplanetary magnetic field strength

On the basis of a consideration of Bartels' historical u index of geomagnetic activity, we devise an equivalent index that we refer to as the interdiurnal variability ( IDV ). The IDV index has the interesting and useful property of being highly correlated with the strength of the interplanetary magnetic field ( B ; R 2 = 0.75) and essentially unaffected by the solar wind speed ( V ; R 2 = 0.01) as measured by spacecraft. This enables us to obtain the variation of B from 1872 to the present, providing an independent check on previously reported results for the evolution of this parameter. We find that solar cycle average B increased by ∼25% from the 1900s to the 1950s and has been lower since. If predictions for a small solar cycle 24 bear out, solar cycle average B will return to levels of ∼100 years ago during the coming cycle(s).