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aa Availability

Copyright WDC C1 for STP at RAL.

The aa index comes from a request made by the Royal Society of London at the Madrid IAGA in 1969 by S. Chapman for Ci before 1884. Early records from Greenwich and Melbourne give a long series of data. Where early individual data records are missing other stations provide approximate information instead.

Current data coverage runs from 1868 through to 1996.