SUNSPOT INDEX DATA CENTER (SIDC)

Director: Ronald Van der Linden

1. Introduction and objectives of SIDC

Sunspots are extended regions on the Sun with a strong magnetic field. They have a lower temperature (3500-4500 K) than the surrounding photosphere (5800 K). The sunspots radiate less energy than the undisturbed photosphere of the Sun and are therefore visible as dark spots on the surface of the Sun.Sunspots are observed with some regularity since 1700 and on a strict daily basis since 1849; the relative number Sunspot number (defined as ten times the number of groups + the number of spots) shows an 11 year cycle detected by Schwabe in 1843. The sunspot number reflects the magnetic activity of the Sun, which has a large impact to the magnetosphere of the Earth and is responsible for e.g. magnetic storms, polar lights. The objective of the SIDC is to carry out and to collect observations of sunspots, to compute, to distribute and to forecast the INTERNATIONAL SUNSPOT NUMBER (Ri).

See a sunspot photograph

2. Short historical account

The Sunspot Index Data Centre was founded in 1981 to continue the work of the Zürich Observatory. With André Koeckelenberg as first director, the Sunspot Index Data Centre started in January 1981 the computation and the diffusion of the INTERNATIONAL SUNSPOT NUMBER (Ri), of which the continuity and coherence with the former Zürich index Rz was insured by the use of the Specola Solare Ticinese Locarno as reference station. The main task of the new centre was to compute and to broadcast the daily, monthly , yearly international sunspot numbers, with middle range predictions (up to 12 months). This task has been successfully fulfilled since 1981 every 1st of the month. The SIDC is also known as World Data Centre C1 for Sunspot index.

3. Observational network

The evolution of the number of cooperating stations to the provisional International Sunspot Number (see table 1) during its 15 years of existence shows that a stabilization has occured after a constant increase in the first years. It is also the case for the contributing network to the definitive International Sunspot Number, and the sub-networks for the provisional and definitive North and South Sunspot Numbers, respectively stabilized at 90, 28 and 50 contributors. The geographical distribution for the definitive sunspot number network is 10% for Belgium, 59% for other European countries, and 31% for the rest of the world.

The cooperating observatories to the International Sunspot Number are (1994):

Americana (Brazil); Athens Nat. Obs. (Greece); Athens by ussps 32404 (Greece); Attikis Nat. Obs. (Greece); Auckland (New-Zealand); Australian Obs. (Australia); Berlin (Germany); Beyazit (Turkey); Bodenteich (Germany); Boom (Belgium); Boulder by ussps 2041 (U.S.A.); Bruxelles-Uccle (Belgium); Bucharest (Rumania); Budapest (Hungary); Buenos-Aires (3 stat. Argentina); Catania (Italy); Ciney (Belgium); Cochamba (2 stat. Bolivia); Cottbus (Germany); Culgoora by ussps 85303 (Australia); Dinant (Belgium); Donostia (Spain); Dover (U.K.); Eilenburg (Germany); Grimbergen (Belgium); Helwan (Egypt); Holloman by ussps 21305 (U.S.A.); Hurbanovo (Slovakia); Invercargill (2 stat. New-Zealand); Inzernhagen (Germany); Jambol (Bulgaria); Jordanston (North Ireland); Jeddah (Saudi Arabia); Kandilli (Turkey); Kanzelhöhe (Austria); Karjali (Bulgaria); Kawaguchi-Saitama (Japan); Kislovodsk (Russia); Langemark (Belgium); Larissa Obs. (Greece); Learmonth by ussps 81202 (Australia); Locarno (Switzerland); Lormont (France); Lunping (Taiwan); Marcq en Baroeul (France); Mons (Belgium); Mosta (Malta); Naxxar (Malta); Etalle (Belgium); Nijmegen (The Netherlands); Mie-Ken (Japan); Ondrejov (Czech Republic); Palea-Penteli (Greece); Palhua by ussps 26204 (Hawai); Potsdam (Germany); Prades (France); Pulligny (France); Pyong Yang (North Korea); Quezon-City (Phillipines); Ramey by ussps 17201 (Puerto-Rico); Roma (Italy); Ronse-Renaix (Belgium); Roquetas-Tortosa (Spain); Rudolstadt (Germany); San Jose (Argentina); San Miguel (Argentina); Santander (Spain); Santiago (Chile); Sliven (Bulgaria); Smolian (Bulgaria); Sobota (Slovakia); Suwa City (Japan); Taipei Obs. (Taiwan); Taipei Weather Bureau (Taiwan); Tilburg (The Netherlands); Tokyo-Mitaka (Japan); Tokyo Nat. Science Museum (Japan); Trieste (Italy); Urawa-Saitama (Japan); Valencia (3 stat. Spain); Vedrin (Belgium); Watukosek (Indonesia); Wittelsheim (France); Zagora (Bulgaria).

Table 1: Number of stations cooperating to the provisional International Sunspot Number

4. Main data provided

The following data are sent to a primary network (more than 80 fax and e-mail) on the first of the month around 10h UT, and to a more extended customers network (460 correspondents) around the 5th of the month (see sec. 3.2);

Moreover, the Centre is also in charge of computing once a year, for the Quarterly Bulletin on Solar Activity, central zone sunspot numbers and averaged spotted areas. It also computes a daily Prompt Photometric Sunspot Index, based on spotted area data. The Centre provides also tables of the positions and evolution of the sunspot groups, per solar rotation.

4.1 The SIDC data base

The following ASCII files from SIDC data base are available and updated every 1st of the month (definitive if already calculated, provisional if not):

Four graphs showing those quantities are also available in the database and are updated every month. The data base also contains the monthly e-mail messages sent to the primary network of correspondents, which are maintained during two months: mai1xxyy.dat, mai2xxyy.dat, xx=month, yy=year. All these data can be accessed by anonymous ftp on ftpserver.oma.be, directory pub/astro/sidcdata (to be changed soon to dist/astro/sidc) and through the World Wide Web, at the adress http://www.oma.be/KSB-ORB/SIDC/index.html

Figure 1 shows the monthly sunspot numbers and the smoothed sunspot numbers for the last four cycles (19,20,21,22) up to August 1996, with 12 months ahead predictions. Note the apparent similarity between the last two cycles. The similarity disappears when considering the North and South contributions, of which the respective maxima are much more separated in cycle 22 than in cycle 21 (Figure 2).

Fig. 1: Monthly and monthly smoothed sunspot number, from 1954 up to August 1996, with predictions.

Fig. 2: Monthly smoothed hemispheric sunspot number from 1954 up to 1996

4.2 Communications network and publications

On the 1st of every month, the computed provisional daily international sunspot numbers for the preceding month and  the 12 month predictions based on Waldmeier's standard curves are broadcasted to a primary network of about 70 correspondents around 10h UT, by fax or e-mail. The message containing the North and South sunspot numbers is sent by e-mail generally one hour later. At the same time, both messages are made available on Internet (anonymous ftp and web).

The computed total, North and South Sunspot Numbers, their monthly means and the predictions also appear in the first and second pages of the monthly publication Sunspot Bulletin, which is sent to a secondary customers network of 450 individuals and institutions early in the month. This bulletin also provides a graph of the smoothed Sunspot number and its predictions for the last 11 years. The third page contains information on the sunspots as observed in Uccle. The fourth page proposes a Monthly summary of Ursigrams giving the provisional values of the SIDC Prompt Photometric Solar Index (PPSI - estimated quantity of flux to be subtracted from the mean solar constant due to sunspots), the radio fluxes on 600 MHz (from Humain-Belgium, radio-astronomical station of the Royal Observatory) and 2800 MHz (Penticton, data furnished by the National Research Council, Canada), the cosmic rays count from the Kerguelen, two SIDC indices on solar X-flux (a Solar Flare Index, SFE -formerly called XFI- and an X-Flux Index X/I), the planetary geomagnetic index from Wingst, the SEA's observed at Uccle and Humain, the magnetic perturbations (added in 1991) detected at the station DOURBES (Royal Meteorological Institute of Belgium) and the principal phenomena associated with solar activity.

The definitve sunspot numbers are published quarterly in SIDC News, together with comments related to solar activity, predictions, etc. The Sunspot Numbers are also sent to Solar-Geophysical Data for publication.

5. Calculating the International Sunspot Number

5.1 The Provisional International Sunspot Number Ri'

The coefficient of the reference station LOCARNO, with respect to the International Sunspot Number is fixed at the value Kloc=0.6, which was the value already used when Zürich was in charge of the sunspot number. Kloc=0.6 is in fact the secular conversion factor of the Swiss stations Zürich, Arosa and Locarno to the former sequence of observations made by Wolf and Brünner after 1820 with a primary Fraunhofer refractor of 70 mm aperture. The daily Ri's are monthly calculated the following way:

For every station, the daily reduction coefficients to LOCARNO are calculated and monthly averaged. The standard deviations are calculated and a first elimination of data is worked out using a 2-sigma criterion. Using these coefficients, daily reduced sunspot numbers are computed, together with their standard deviation. New monthly means and monthly standard deviations of the reduction factors are computed, after elimination of the daily factors deviating of more than the standard deviation.

Elimination using a 1-sigma criterion is again used on newly calculated daily means, until the number of retained stations remains unchanged, or the final relative standard deviation is lower than 10%. The last result is the daily International Provisional Sunspot Number Ri', except when it is nearly zero. In this case, a special procedure is used, taking into account that when a significant fraction of the stations has non-zero values, it is accepted that Ri' does not vanish, and only those stations who have observed something are retained in its constitution.

5.2 The Definitive Sunspot Number Ri

Generally, about 90 stations are available for the calculation of the definitive sunspot number; the same method is used as for the provisional one. The provisional and definitive sunspot numbers are then daily compared. When they do not differ from each other by more than 5%, the provisional value is accepted as definitive. Otherwise, the definitive value is adopted, unless the daily histogram of the sunspot numbers shows an anomalous distribution, in which case a new value is calculated after elimination of the too discrodant stations.

5.3 The Hemispheric Sunspot Numbers Rn and Rs

Those new data are provided since January 1992. They are calculated in the same way as the total sunspot numbers, separately for both hemispheres. LOCARNO is here also the reference station. The results are normalized to the International Sunspot Number, in order to satisfy the relation Rn + Rs = Ri. Definitive values calculations use around 50 stations.

5.4 Forecasting of the unsmoothed sunspot number

The principle of the method is that the last smoothed sunspot number known defines by interpolation a specific curve in a family of Waldmeier's standard curves. The eighteen monthly points following the last smoothed sunspot number on his interpolated curve define then predictions up to 12 months from the current date. This is the so-called "classical method" from which the "SIDC adjusted" method was derived to take more into account the trend of the preceeding months. However, this last method has not brought significant improvement, and will be replaced in 1997 by a new forecasting method due to K. Denkmayr which is essentially a non-parameteric regression-technique coupling a dynamo-based relation with Waldmeir's idea of standard curves.

6. Research work at the SIDC

6.1 Quality control of the data

This control essentially consists in a permanent comparison between a sub-group of 21 reliable stations (including the reference station LOCARNO) and the International Sunspot Number Ri in order to

1. detect possible long term discrepancies (for example systematic drifts) in Ri
2. select additional reference stations, in order to minimize in the future such possible effects and ensure a good continuity in the computation of Ri

6.2 More general research

6.2.1 Sunspot index related research:

• statistical, Fourier and wavelet analysis of the daily, monthly and yearly Sunspot Numbers (in relation with the solar rotation, the 11 year cycle, super-cycle at 80-100 years, etc);
• Research about the methods of predictions;
• Study about the asymetric behaviour of the hemispheric sunspot numbers;
• Publication of the tables Position and Evolution of solar sunspot groups;

6.2.2 The Centre is closely associated with the department of radio-astronomy and solar physics of the Royal Observatory of Belgium, of which the field of research involves solar coronal structure observations (eclipses, EIT-SOHO observations) and modelling, photospheric and chromospheric observations.

Most of the research work quoted above implies international collaborations.

Back to the FAGS home page

For further information link to the SIDC home page http://www.sidc.oma.be
or contact:

Dr. Ronald Van der Linden
Director SIDC
Observatoire Royal de Belgique
Ringlaan 3
B-1180 BRUXELLES
Belgium
TEL: +32(0)2-3730491
FAX: +32(0)2-3730224
e-mail: ronald.vanderlinden@oma.be