| This file is comprised of absolute mean hourly values of the geomagnetic components
D, H and Z or X, Y and Z. These values, on magnetic tape, are available from approximately
220 of the 400 worldwide observatories which have ever operated. Most of the values
were hand-scaled from the analog recordings (magnetograms) and were added to the file
via punched cards. From some observations, for more recent years, there are means
of values recorded directly on magnetic tape from digital magnetometers at various
sample rates, for example, each minute. The accuracy of these data varies but is generally
good, particularly for the more recent years. Approximately 60 observation-years of
these data are added to the files each year. |