**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and Letfus **(unpublished), **Knoska **and **Petrasek** (1984),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 **(**N**ational **G**eophysical
**D**ata
**C**enter).
It is calculated for each using the formula:

- where "i" is the

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

**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.

Author: Tamer Ataç

**Flare Index Data
**1976-2014 (catalog.zip)

24.
Solar Cycle

Solar Flare
Theory |

Daily Solar Flares Images :
Ondrejov
Observatory

**Mailing Address: **
Boğaziçi University Kandilli Observatory and Earthquake Research Institute,
Çengelköy 34684 İstanbul / TURKEY