What is Flare Index?

A solar flare is an enormous explosion in the solar atmosphere which is defined as a sudden, rapid and intense variation in brightness. It is beleived to result from the sudden release of energy stored in magnetic fields. The flare index is of value as a measure of this short-lived activity on the Sun.

Kleczek (1952) first introduced the quantity "Q = i x t" to quantify the daily flare activity over 24 hours per day. He assumed that this relationship gives roughly the total energy emitted by the flares. In this relation, "i" represents the intensity scale of importance and "t" the duration (in minutes) of the flare. Some reviews of flare activity using Kleczek's method are given for each day from 1936 to 1997 by Kleczek (1952), Knoska and Letfus (unpublished), Knoska and Petrasek (1984), Atac (1987) and Atac and Ozguc (1986-2000). The daily flare index for the 21, 22, and 23 st Solar Cycles were determined by using the final grouped solar flares which are compiled by NGDC (National Geophysical Data Center). It is calculated for each using the formula:

 

Table 1

Importance           i   I	Importance           i
SF, SN, SB        0.5	2B                       2.5
1F, 1N               1.0	3N, 3F, 4F          3.0
1B                      1.5	3B, 4N                3.5
2F, 2N               2.0	4B                      4.0

To obtain final daily values, the daily sums of the index for the northern and southern hemispheres and for the total surface are divided by the total time observation of that day calculated from Solar-Geophysical Data, Comprehensive Reports solar flare list.
Average optical importance of flare is a measurement of flare size and brillance. The first character in the importance ranks is the area of the brightened plage on a 5-level scale  and the second character grades the plage brilliance on a 3-level scale. See the table below. Under this classification scheme, an SF event is the smallest and faintest kind of flare, and a 4b event is the largest and brightest.
                                 

                               Flare Area       Flare Brillance
                Imp             Actual^*             Apparent^*
                -----------------------------------------------------------------------------------------------
                S                         A <   2.0                         A <    200                         f, n, b^**
                1                2.1 < A <   5.1               200 < A <   500                          f, n, b
                2                5.1 < A < 12.4               500 < A < 1200                          f, n, b
                3              12.5 < A < 24.7             1200 < A < 2400                          f, n, b
                4                         A > 24.7                          A > 2400                          f, n, b
               ------------------------------------------------------------------------------------------------
                 * Actual area in units of square degrees. Apparent area in millionths of solar disk.
               ** f= faint, n=normal, b=bright

NGDC compute the average optical importance of a flare by substituting numerical values for the area and the brillance rankings of each report and calculating the arithmetic mean of each part. The mean of the area classes and the mean of the brightness classes are then converted to their respective 5- and 3- level scales.

 

It is recommended that it would be appropriate courtesy to acknowledge  in publications that the flare index data were calculated by the staff of Kandilli Observatory, and to send a preprint or reprint. The following statement is suggested: "Flare Index Data used in this study were calculated by T.Atac and A.Ozguc from Bogazici University Kandilli Observatory, Istanbul, Turkey"

REFERENCES

Atac, T.: 1987, Astrophys. Space Sci. 135, 201.
Ozguc, A. and Atac, T.: 1989, Solar Phys., 123, 357-365
Ozguc, A. and Atac, T.: 1994, Solar Phys., 150, 339-346
Ozguc, A. and Atac, T.: 1996, Solar Phys., 163, 183-191
Atac, T.,and Ozguc, A.: 1996, Solar Phys., 166, 201-208
Atac, T.,and Ozguc, A.: 1998, Solar Phys, 180, 397-407
Atac, T.,and Ozguc, A.: 2001, Solar Phys. 198, 399-407
Ozguc, A. and Atac, T.: 2003, Solar Phys. 214, 375-396
Kleczek, J.: 1952, Publ. Inst. Centr. Astron., No. 22, Prague.
Knoska, S and Petrasek, J.: 1984, Contr. Astron. Obs. Skalnate Pleso 12, 165.

ACKNOWLEDGEMENTS

The authers would like to thank H.E. Coffey and E.H. Erwin, WDC-A for Solar-Terrestrial Physics, NOAA E/GC, Broadway 325, Boulder, CO, 80303, who made available the grouped flare lists. A digital version of these data are available at ngdc.noaa.gov in the Solar and Upper Atmosphere on-line databases.