Is there a need to revise the aa index?

The three-hourly aa index, retrospectively calculated from 1868 [1], provides one of the longest continuous global geophysical data sets that can be used in the analysis of magnetospheric and ionospheric phenomena. Designed to cancel out systematic LT and annual variations, aa is derived according to: K indices from 2 geomagnetically near- antipodal magnetic observatories equivalent amplitudes (nT) and scale factors to correct for differences in geomagnetic latitude and local induction effects (providing aa and aa ) n s average of aa and aa n s Further LT cancellations can be gained by applying: 8-point (24 hour) running mean (aa*) [2]