Interhourly Variability Index of Geomagnetic Activity and Its Use in Deriving the Long-Term Variation of Solar Wind Speed
Abstract
[1] 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 time scale of a 27-day Bartels rotation, averages for stations with corrected geomagnetic latitude less than 55 degrees, are strongly correlated with midlatitude range indices (R2 - 0.96 for the am index since 1959; R = 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 paremeters (R2 =079 with RV2).
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 31, 2007
- Accession Number
- ADA474310
Entities
People
- Edward W. Cliver
- Leif Svalgaard
Organizations
- Air Force Research Laboratory