The geomagnetic aa indices are the continuation of the series beginning in the year
1868. A full description of these indices is given in the International Association
of Geomagnetism and Aeronomy (IAGA) Bulletin No 33, which contains them for the years
18681967. Descriptions are also given (especially comparisons with am, ap, or Ci
indices) in two short papers [Ann. Geoph., 27, 6270, 1971 and J. Geophys. Res., 77,
68706874, 1972]. The aa values form 19681975 are contained in the IAGA Bulletin
39. From 1976 onward they are included in IAGA Bulletin 32. Revised aa values for
the years 19691976 have been distributed in 1979 to the recipients of IAGA Bulletin
32 in the form of loose sheets to be inserted in the Bulletin 39 (19681975) and 32f
(1976). A graph of these values 1868  1979 is published in the 426 Part I issue of
SolarGeophysical Data (SGD) (page 137). Revised aa values for 19681977 appear in
the 411 Part II February 1977 SGD. Briefly, such threehourly indices as these, computed
from K indices of two antipodal observatories (invariant magnetic latitude 50 degrees),
provide a quantitative characterization of the magnetic activity, which is homogeneous
through the whole series. Halfdaily and daily values give an estimation of the activity
level very close to that obtained with "am" indices. Values are in gammas and correspond
to the activity level at an invariant magnetic latitude of 50 degrees. The aa indices
are computed for: N = daily values for the Northern Hemisphere, S = daily values for
the Southern Hemisphere, M = halfdaily values of aa indices for the Greenwich day.
Letters C and K refer to a classification of the quiet days of the month (C = really
quiet, K = quiet but with slightly disturbed threehourly intervals). The letters
on the left refer to the 24hour Greenwich day, on the right to a period of 48 hours
centered on the Greenwich noon. The threehourly indices aa are available from the
appropriate World Data Centers in digital form using the format described in IAGA
Bulletin 33.
