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NOTE: The storm sudden commencement data file was updated on 18 Feb 1998.

o Revised data for 1968-1975 were taken from IAGA Bulletin No. 39,
SUPPLEMENTARY GEOMAGNETIC DATA 1957-1975, by P.N. Mayaud and A. Romana,
IUGG Publications Office, 1977.
o SSC amplitudes and duration data were added to 1976-1989 data. These
data were also updated using the IAGA Bulletins No. 32 series which
contain the final IAGA data. In some cases, SSCs in the earlier list
disappeared (see explanation for change in the definition in text below).
o Data after 1989 are still preliminary.





TEXT EXCERPTED FROM pages 23-24, IAGA Bulletin No. 32t, GEOMAGNETIC DATA 1989,
IAGA INDICES: aa, am, Kp, Dst, AE, RAPID VARIATIONS, edited by Annick
Berthelier and Michel Menvielle, ISGI Publications Office, 4 Avenue de
Neptune, F-94107, Saint Maur des Fosses Cedex, France, 1996:


1.5.RAPID VARIATIONS

1.5.1.Storm sudden commencements (ssc)

The old definition, that said that "an ssc is a sudden commencement of a
magnetic storm", is now changed into "sudden commencements followed by a
magnetic storm or by an increase in activity lasting at least one hour".

The change was introduced by P.N. Mayaud (LAGA Bulletin 33 : "A Hundred Years
Series of Geomagnetic Data, 1868-1967. Indices aa and storm sudden
commencements"; see also IAGA Bulletin 39: " Supplementary Geomagnetic Data
1957-1975", including a new list of ssc's 1968-1975). In this new definition
more importance is given to the change of rhythm in the magnetic activity,
than to the amplitude of the magnetic storm, and therefore some ssc's
included in the list are not followed by a real magnetic storm.

The routine procedure of the Service in establishing the final list consists
essentially of several steps, following as much as possible the instructions
given by Mayaud in order to get a continuous series of homogeneous data.

Step 1. From the monthly reports of Rapid Variations prepared by 43 magnetic
observatories (see list page 121), a combined list of all events is
compiled.

Step 2. This list is checked against the magnetograms of two low latitude
observatories (HON and EBR), and cases reported by only a very few
observatories and that are clearly not an ssc in the magnetograms of
HON and EBR, are eliminated.

Step 3. All other cases are then include in a new list, and copies of the
magnetograms for all of them are requested from the five collaborating
low latitude observatories, that are regularly spaced in longitude
(Table 5a), or from their supplementary observatories (Table 5b).

Step 4. From the copies of the magnetograms sent by the five observatories
as a reply to Step 3, every ssc is evaluated independently for each
observatory and a number (0 to 3) is assigned to each event, in the
following scale:

0. the event could have escaped the attention of the observers or
does not deserve to be called an ssc,
1. the event is possibly an ssc, but is not clear enough in itself
to be identified as a true ssc without inspection of other
records at different longitudes,
2. the event can be unmistakably identified as an ssc from the
single record on consideration.
3. is assigned, instead of 2, when the event presents a very sharp
change of rhythm in the agitation, has a large amplitude and
its general morphology is remarkable.

If an event gets three codes 0 out of the five observatories, it is rejected
from the list.

Step 5. Finally the duration in minutes, and the amplitude in nT are measured
on the copies from the five observatories and their mean values are
calculated.


TABLE 5

a - Collaborating Observatories for Rapid Variations

Observatory Symbol Geographic Geomagnetic
N Lat E Long N Lat E Long
==================================================================
MBOUR MBO 14.40 343.02 20.68 56.80
FUQUENE FUQ 5.47 286.27 16.47 357.07
HONOLULU HON 21.32 202.00 21.46 268.57
PORT MORESBY PMG - 9.40 147.15 -17.99 219.75
ALIBAG ABG 18.63 72.87 9.64 145.39


b - Supplementary Observatories

Observatory Symbol Geographic Geomagnetic
N Lat E Long N Lat E Long
=================================================================
TENERIFE SZT 28.48 343.72 34.60 60.30
SAN JUAN SJG 18.38 293.88 29.36 5.21
APIA API -13.80 188.22 -15.61 261.99
KANOYA KNY 34.42 130.88 21.12 199.85
HYDERABAD HYB 17.42 78.55 7.86 150.69


In the list of storm sudden commencements (section 5.1., pages 121-123) the
following indications are given for each event:

a) Day, hour and minute (mean value taken from the monthly reports);
first and last minutes thus reported are given in brackets at the end
of the line.
b) The five number codes (one for each collaborating observatory); if
some figures are underlined, it means that the replacement observatory
was used.
c) The mean average duration, in minutes, of the event and its average
amplitude, in nT.
d) The following figures indicate the number of observatories that in
the monthly reports have characterized the ssc by each of the three
letters A, B, or C, their meaning being:
A = very remarkable
B = fair, ordinary but unmistakable
C = very poor, doubtful,
Please note that the 0 to 3 qualification is given by the Service on Rapid
Variations, independently from these A, B, C, qualifications which are given
by the observatories, although some sort of correlation must exist among them.

The last group indicates the number of observatories that have classified the
event other than an ssc, namely:
si = sudden impulse, but not an ssc;
sfe = solar flare effect
b = bay-like disturbance.