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MAJOR MAGNETIC STORMS 1932-1997
ACCORDING TO THE Ap* CRITERIA


To view a plot of the number of magnetic storms v.s. sunspots,
go to ftp://ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_DATA/APSTAR/magstorm.gif

A simpler global index of magnetic activity (other than Kp
or Km) 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 observatories. 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 the 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. Before that (back to 1868), the range of variation between most
disturbed and least disturbed years was not so great as is common for cycles
since the mid-1900's. 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. A look at several examples clearly showed that magnetic storms
often begin in any of the 3-hour intervals into which the UT-day is divided.
Those that have their most disturbed 24-hour interval beginning and ending with
the UT-day produce the largest Ap valued events. At WDC-A for STP, a
now-routine procedure is to take the 3-hourly ap indices when they are received
and compute an 8-point running average. When this mean exceeds 40, 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 40. Then the maximum mean value attained is designated Ap*.
When the most disturbed 24-hours begin at the start of the UT-day, the values
of Ap and Ap* are identical. From experience, it appears that at least 7/8th
of the major magnetic storm events have a value of Ap* that exceeds the
largest value of Ap (as one might expect).

Ap* is the maximum average 24-hour global disturbance based on the
linear ap indices for each major magnetic storm and has been tabulated at
WDC-A for STP since it was introduced by Joe Allen in 1974. Graphs and
tables of these indices and explanations of their simple derivation published
in various papers and proceedings beginning with the list of key dates included
in "Solar-Terrestrial Physics and Meteorology: A Working Document," Shapley,
A.H., H.W. Kroehl, and J.H. Allen, SCOSTEP, July 1975. The computer output
printed following this introduction is summarized for each month and year since
1932, the year for which Kp indices are first available. A few time series
are plotted by computer to illustrate the bimodal annual distribution of the
major magnetic storms with occurrence peaks around each equinox, the relatively
uniform distribution of peak disturbance onset times across the UT-day,
and the relatively different distributions in each solar cycle between the
number of major magnetic storms per year and annual sunspot numbers.

J. H. Allen, 94/01/20

Download the Ap* Storm List: ftp://ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_DATA/APSTAR