IHV: a new long-term geomagnetic index Original Research Article

We derive a new daily index of geomagnetic activity, the Inter-Hour Variability index (IHV), for investigations of the long-term variability of the solar wind-magnetosphere system. The IHV index is used to successfully reconstruct yearly-averages of the range indices am, ap, and aa from 1959 through 2000. When we attempt to reconstruct the aa index back to 1901, however, the reconstructed aa lies above the observed aa for years before 1957, with the difference between the two curves being the greatest (∼5–10 nT) during the first two decades of the 20th century.