For a plot of sunspot number and aa Yearly Indices 1868-2005,

A simple global index of magnetic activity is produced in France from the K
indices of two nearly antipodal magnetic observatories in England and
Australia.  This index aa, is the 3-hourly equivalent amplitude antipodal
index.  Daily average AA may be derived similarly to Ap.  An historical
advantage to using aa is that these indices have been extended back in time
through scaling of magnetic activity from magnetograms of earlier
observations.  The aa indices are derived from 1868 to the present.  An AA*
index has been derived that is the counterpart of Ap* but is available for
a longer span of years.  Because of the difference in units of
presentation, the values of AA* and Ap* are not the same so that different
major magnetic storm onset and end threshold values are used for the two
series.  However their comparison for the years of overlapping coverage
show that relative frequency of occurrence of major storms per year are
similar.  Another reason for differences is that an index derived from
magnetic perturbation values at only two observatories easily experiences
larger extreme values if either input site is well situated to the overhead
ionospheric and.or field aligned current systems producing the magnetic
storm effects.  Although not documented here, it is interesting to note
that the overall level of magnetic disturbance from year to year has
increased substantially from a low around 1900.  Also, the level of mean
yearly aa is now much higher so that a year of minimum magnetic
disturbances now is typically more disturbed than years at maximum
disturbance levels before 1900.

Examination of the annual distribution of Ap indices revealed UT-days when
elevated, clearly magnetic storm values, were encountered on successive
days.  This same distribution would be showed in the aa indices.  At WDC-A
for STP, a now-routine procedure is to take the 3-hourly aa indices when
they are received and compute an 8-point running average.  When this mean
exceeds 60, a "major magnetic" storm is considered to be in progress.  It
is arbitrarily considered to continue in progress until such time as the
value drops below a threshold of 60.  Then the maximum mean value attained
is designated AA*.

AA* is the maximum average 24-hour global disturbance based on linear aa
indices for each major magnetic storm and has been generated at WDC-A for
STP since Les Morris modified the Ap* program that was developed by Joe
Allen in 1974.

To obtain the latest AA* listing of Major Magnetic Storms via FTP
go to