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We describe the detailed derivation of the interhourly variability ( IHV ) index of geomagnetic activity. The  IHV  index for a given geomagnetic element is mechanically derived from hourly values or means as the sum of the unsigned differences between adjacent hours over a 7‐hour interval centered on local midnight. The index is derived separately for stations in both hemispheres within six longitude sectors spanning the Earth using only local night hours. It is intended as a long‐term index and available data allows derivation of the index back well into the 19th century. On a timescale of a 27‐day Bartels rotation,  IHV  averages for stations with corrected geomagnetic latitude less than 55° are strongly correlated with midlatitude range indices (R 2  = 0.96 for the  am  index since 1959; R 2  = 0.95 for the  aa  index since 1980). We find that observed yearly averages of  aa  before the year 1957 are ∼3 nT too small compared to values calculated from  IHV  using the regression constants based on 1980–2004. We interpret this discrepancy as an indication that the calibration of the  aa  index is in error before 1957. There is no systematic discrepancy between observed and similarly calculated  ap  values back to 1932. Bartels rotation averages of  IHV  are also strongly correlated with solar wind parameters (R 2  = 0.79 with  BV  2 ). On a timescale of a year combining the  IHV  index (giving  BV  2  with R 2  = 0.93) and the recently developed interdiurnal variability ( IDV ) index (giving interplanetary magnetic field magnitude,  B , with R 2  = 0.74) allows determination of solar wind speed,  V , from 1890 to present. Over the ∼120‐year series, the yearly mean solar wind speed varied from a low (inferred) of 303 km/s in 1902 to a high (observed) value of 545 km/s in 2003. The calculated yearly values of the product  BV  using  B  and  V  separately derived from  IDV  and  IHV  agree quantitatively with (completely independent)  BV  values derived from the amplitude of the diurnal variation of the horizontal component in the polar caps since 1926 (and sporadically further back).

Interhourly variability index of geomagnetic activity and its use in deriving the long‐term variation of solar wind speed