Copyright WDC C1 for STP at RAL.
am is a planetary index of geomagnetic variation measured at several longitudes round the globe. Both daily and three-hourly data are available here at RAL. The am data's average am data indices have been derived by several different groups. The daily values are formed from an average of the 8 three-hourly values.
The am index was devised by Bartels and agreed by IAGA in Madrid in 1969. The am index records begin in 1959 and continue through to December 1996.