Since 1975, Ebre Observatory houses the International Service on Rapid Magnetic Variations, responsible for creating and publishing the lists of rapid variations. This task was entrusted by the International Association of Geomagnetism and Aeronomy (IAGA), and is part of the International Service of Geomagnetic Indices (ISGI).
The geomagnetic field exhibits variations due to different phenomena. It has regular variations (the clearest example is the daily variation) and irregular variations (such as geomagnetic storms), considered in this sense as magnetic disturbances. Often, the latter have a sudden onset, so we call them rapid variations. The study of rapid variations has been difficult, especially at the time of establishing a definition of the phenomenon and giving their explanation. The phenomena studied are SC (sudden commencement), which can be SSC (storm sudden commencement) if it is a subsequent magnetic disturbance, or SI (sudden impulse) if no further disturbance, and SFE (magnetic effect of a solar flare).
The following are the final annual lists of rapid variations that can be found in the IAGA bulletins. It has been developed a list by year in which the SC’s and SFE’s are classified. From the year 2015 on, presentation of the preliminary data of the rapid magnetic variations will be similar to those of the definitive ones. That is, the SSC and sfe data will appear separated. The name of the archives with the preliminary data of every year will be similar to the one of the definitive data, but with a final "p" instead of "d" of the definitive ones.
The given tables of data correspond to different series. The first corresponds to the period 1868-1967, the second to the period 1968-1994, the third to the period 1995-2005, and the last one to the period beginning the year 2006. In all of them qualification codes are given of the SC records of low latitude observatories selected by IAGA for the determination of the data of the SC. The codes have practically the same meaning in all series, which is the following one:
3 or 2: the event can be unmistakably identified as being an SC from the single record under consideration. Number 3 is imposed instead of 2 when the following three features are present: a) very sharp change of rhythm, b) large amplitude of the sudden movement, c) remarkably morphology of it
1: means that the event seen in this particular record is possibly an SC, but it is not sufficiently clear by itself for stating that it is a true SC: One needs records from other longitudes for getting a firm judgement
0: means that the event, such as it appears in the record, could escape from the attention of the observer, or does not deserve to be called an SC. From 2006 it can mean that some characteristic defining the SC, fails
Tables for the period 1868-1967
Data of the 100 first years (1868-1967) have been obtained by P.N. Mayaud from three collections of records that he calls: N (mainly containing records of Saint-Maur, Val-Joyeux and Chambon-la Foret), S (fundamentally with records of Melbourne y Toolangi) and T (with records of Colaba y Alibag fundamentally, including records of Batavia-Buitenzorg-Kuyper-Tangerang from1880).
According to Mayaud this table contains only SSC data. He believes that a SSC exist when there is a change of rhythm in the disturbance before and after the sudden impulse, even if the change is very small. If there is no that change this event would be a SI.
For every SSC the tables contain the date (DATE), the time (HH:MM), the qualification codes corresponding to each collection N, T and S (CODE), the SSC duration in minuts (DURATION), its amplitude in nT in the records of collection T (AT), and the normalized amplitude of one of the other two collections or the normalized mean of both (ANS). (cf. IAGA Bulletin No. 33)
Tables for the period 1968-1994
The data are obtained from the records of the five low latitude observatories: MBO, FUQ, HON, PMG and ABG. When the record of some of these observatories is missing the record of the corresponding supplementary observatory is used. The supplementary observatories are GUI, SJG, API, KNY and HYB respectively.
For every SSC the tables contain the date, the time, the qualification codes of the records of the five observatories, the SSC duration, its amplitude, the qualification codes given by the collaborating observatories of their records, and the number of observatories that qualify the event as Si or sfe. The SSC time, duration and amplitude are the mean values obtained from the records of the five low latitude observatories. The Qualification codes used by the collaborating observatories have the following correspondence with the codes defined above: A corresponds to 3, B to 2 and C to 1.
A hyphen indicates that there is no value.
Tables for the period 1995-2005
The column with the qualifying codes given by the collaborating observatories has been omitted. The list of the collaborating observatories has been added before the tables. The penultimate column indicates the qualification of the events in the records of the five low latitude observatories reported in the last column.
Tables from 2006
According with the decision taken in the XXIV IUGG General Assembly at Perugia in 2007, the following changes have been introduce in the treatment of rapid magnetic variation data from 2006.
The presentation of the SSC lists has been changed. Now, events are named as SC. In the last column of the tables, data are differentiated as SSC (Storm Sudden Commencement) or SI (Sudden Impulse). SSC are those SC followed by an increase of magnetic activity within the next 48 hours. If there is no increase of magnetic activity, then it is an SI. For an SC to be considered as SSC, the threshold of the magnetic activity increase is reached when the Dst index diminishes to, at least, -50 nT, or when the kp index increases to, at least, 5o. Moreover, for an event to be considered as an SC the rate of change of a sudden increase of the magnetic field should be, at least, 3 nT/min.
In November 2010 the values of the SSC initial minute, duration and amplitude corresponding to the years 2000 through 2004 were corrected because several errors have been detected. Most of the errors of the SSC initial minute and duration do not exceeded one minute, except for 3 cases of the initial minutes and 2 cases of duration. With respect to the amplitude errors, only 4 cases exceeded 2 nT.
Tables from the year 1994 have the same format as before, but since then they are written in text format to facilitate the use of the data.
There are two lists for every year of this period: One of them gives the date and time of the clear sfe, and the second one gives the same data of the considered doubtful sfe.
The meaning of the classifying characters of the sfe during this period is as follows:
1.- The letters preceding the series of Observatories reporting any event means the answer of the involved observatories to the question: "do the magnetic records show a movement at the time given ?", and have the following meaning:
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
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.
2.- The meaning of the numbers that appear after the IAGA codes of each observatory is:
3 = the observed movement is certainly a sfe
2 = the observed movement is probably a sfe
1 = the observed movement probably is not a sfe
0 = the observed movement certainly is not a sfe
3.- Stations in the twilight zone, reporting a movement, are indicated by normal brackets; those in the night-side of the earth are indicated by square brackets.
4.- The letter C preceding the date of the sfe means that it has been confirmed by ionospheric or solar data.
SFE Tables from 1995
Sfe "qualification" meaning
1 - Doubtful sfe
2 - The shape and extension of the phenomenon correspond to a sfe
3 - The shape and extension of the phenomenon correspond to a notable sfe