Long-term geomagnetic indices and their use in inferring solar wind parameters in the past Original Research Article

We discuss three new geomagnetic indices [the Inter-Hour Variability (IHV), the Inter-Diurnal Variability (IDV), Svalgaard, L., Cliver, E.W. The IDV index: its derivation and use in inferring long-term variations of the interplanetary magnetic field strength. J. Geophys. Res. 110, A12103. , 2005; and the Polar Cap Potential (PCP) index, Le Sager, P., Svalgaard, L. No increase of the interplanetary electric field since 1926. J. Geophys. Res. 109 (A7), A07106. , 2004], that are derivable from data available for a century or more. Each of these indices responds directly to either the solar wind magnetic field strength (B) or to different combinations of B and the solar wind speed (V). This over-determined system permits a reconstruction of these parameters for the past ∼150 years. The variation of yearly averages of B can be described as a constant value (4.6 nT) plus a component varying with the square root of the sunspot number. Because the latter seems to exhibit a ∼100 year Gleissberg cycle, B does as well. Since 1890, annual averages of V range from a low of ∼300 km/s in 1902 to 545 km/s in 2003. The IHV-index fords a way to check the calibration of other long-term geomagnetic indices. We find that the ap-index tracks the variation of IHV, back to 1932 but that the aa-index (extended back to 1844) is systematically too low (3–6 nT) before 1957, relative to modern values.