IAGA
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| ssc-Sfe |
Rapid Variations
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 (IAGA 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 45 magnetic observatories, 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 included 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 :
|
Observatory |
Symbol |
Geographic |
Geomagnetic |
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|
Lat. (°N) |
Long. (°E) |
Lat. (°N) |
Long. (°E) |
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|
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 |
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PORT MORESBY |
PMG |
-9.40 |
147.15 |
-17.99 |
219.75 |
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ALIBAG |
ABG |
18.63 |
72.87 |
9.64 |
145.39 |
or from their supplementary observatories :
|
Observatory |
Symbol |
Geographic |
Geomagnetic |
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|
Lat. (°N) |
Long. (°E) |
Lat. (°N) |
Long. (°E) |
||
|
TENERIFE |
SZT |
28.48 |
343.72 |
34.60 |
60.30 |
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SAN JUAN |
SJG |
18.38 |
293.88 |
29.36 |
5.21 |
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APIA |
API |
-13.80 |
188.22 |
-15.61 |
261.99 |
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KANOYA |
KNY |
31.42 |
130.88 |
21.12 |
199.85 |
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HYDERABAD |
HYB |
17.42 |
78.55 |
7.86 |
150.69 |
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. In the list of storm sudden commencements 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.
The aim of the Service is to provide a reliable list of times when a magnetic solar flare effect did occur. Unfortunately, the reports on solar flare effects are not provided by all the observatories sending regularly the monthly reports on Rapid Magnetic Variations to the Service. As the sfe is a phenomenon dependent of longitude, it is not an easy task to provide a complete round the world coverage.
Step 1. The list of all times given in the monthly reports from observatories as possible occurrences of sfe's are checked with data taken mainly from Solar Geophysical Data, on the occurrence of alpha-flares, radio bursts or SID's, as, by definition, the crochet-like disturbance in the magnetograms must be an effect from solar activity and more precisely of a light emission from the Sun.
Times that are not correlated with any of these solar or ionospheric phenomena are eliminated from the list except in the cases when they were reported by more than one observatory or when a single observatory gave the time with the indication that it was confirmed. These exceptions try to take into account the fact that the correlation between solar events and magnetic crochets is not hundred percent.
Step 2. The list prepared in such a way is sent to all magnetic observatories, as a checking list, and they are asked to inspect their magnetograms and provide their judgement in two steps:
First : Do the magnetic records show a movement at the time given ? The answer is given by the following letters:
A = very clear movement
B = fair, ordinary movement
C = very poor movement
D = movement not observed, although records are satisfactory
E = movement cannot be observed due to heavy disturbance
X = record missing
Note : Please note that this letter index refers only to the existence of a movement in the curves and not to the opinion of this movement being or not being a sfe.
Second : The opinion of this movement being or not a sfe is given after letters A, B, and possibly C, by a number index:
3 = certainly a sfe
2 = probably a sfe
1 = probably not a sfe
0 = certainly not a sfe
The answers from observatories are plotted into world maps for each event ; in these maps are also drawn times showing the sunlit hemisphere and the twilight zones.
Step 3. From these maps a judgement is made about the event being or not a sfe. A simultaneous judgment is done about the existence or not of a solar, ionospheric and radio-sun event that could be the cause of the magnetic disturbance. And finally each event is classified as a sfe, a doubtful sfe or a rejected sfe.
Tables for solar flare effects and for doubtful solar flare effects are given in ssc-sfe .. The format is the following:
a) A summary list gives for each month, the day and time of the beginning of sfe; if the sfe is confirmed the letter C follows the time of the event. Then are indicated the number of observatories having reported a movement (letters A, B, C, see explanation in Step 2 above), and the number of observatories having not reported any movement (letters D, E, X, see Step 2 above).
b) It is followed by detailed tables giving for each event the list of observatories reporting it with the number-index, arranged according to the classifying letter and to longitude.
Stations in the twilight zone, reporting a movement (A, B, C and also E and X) are indicated by normal brackets; those in the night-side of the earth by square brackets. Station in these two zones reporting D have been omitted from the lists.
The separation into such two tables of sfe and "doubtful" sfe may be rather subjective, due to the fact that we do not really know the mechanism linking the radiation emission from a flare and its effect on the magnetic records.
For instance, if we accept that the link is an ionising radiation from a solar flare producing an electric current in some region of the sunlit ionosphere, can this current extend its effect into the night hemisphere of the earth? And can all flares, observed in the alpha-light, produce such ionisation ? We are aware, from observations of solar flux, that not all frequencies produce similar magnetic effects. Satellite observations of X-ray flares may help in solving these and similar questions, but until now we have a fairly good coverage of alpha-flares and a rather limited of X-flares. Moreover it must be taken into account that not all flares have the same emission of energy and that their effects on the earth therefore may be limited to a smaller, or to a larger region of the earth.
Following the same kind of reasoning, we may find different effects due to the geometric relative position of the angle of emission in the Sun and the angle of reception at the earth.
For all these reasons, the tables of solar-flare-effects do not appear to be directly usable for detailed statistical studies, but they may be a good indication for researchers, where to look in the magnetic records when they study some particular event.