RELATIVE SUNSPOT NUMBERS

Description of Data from SGD Explanation of Data Reports: 
Relative Sunspot Numbers -- The relative sunspot number is an index of the 
activity of the entire visible disk of the Sun.  It is determined each day 
without reference to preceding days.  Each isolated cluster of sunspots is 
termed a sunspot group, and it may consist of one or a large number of 
distinct spots whose size can range from 10 or more square degrees of the 
solar surface down to the limit of resolution (e.g., 1/25 square degree).  The 
relative sunspot number is defined as R = K (10g + s), where g is the number 
of sunspot groups and s is the total number of distinct spots.  The scale 
factor K (usually less than unity) depends on the observer and is intended to 
effect the conversion to the scale originated by Wolf.

The provisional daily Zurich relative sunspot numbers, Rz, were based upon 
observations made at Zurich and its two branch stations in Arosa and Locarno 
and communicated by M. Waldmeier of the Swiss Federal Observatory.  Beginning 
January 1, 1981, the Zurich relative sunspot number program is replaced by 
the "Sunspot Index Data Center" (c/o Dr. P. Cugnon, 3 av. Circulaire, B-1180 
Bruxelles, Belgium).  The determination of the provisional International 
Sunspot Numbers Ri results from a statistical treatment of the data 
originating from more than twenty-five observing stations.  These stations 
constitute an international network, with the Locarno (Switzerland) station as 
the reference station, to guarantee continuity with the past Zurich series of 
Rz.  The definitive International Sunspot Numbers Ri are evaluated by a 
similar method based on a network of observing stations selected for their 
high number of observations, their continuity during the whole year and an 
existing series of observations during the last years.  Also taken into 
account is the stability of the K monthly factors with reference to the 
Locarno station.  These relative sunspot numbers are now designated Ri 
(International) instead of Rz (Zurich).

The daily American relative sunspot numbers, Ra, are compiled by Peter O. 
Taylor (4523 Thurston Lane, #5, Madison, WI 53711 USA) for the Solar Division 
of the American Association of Variable Star Observers (AAVSO).  The Ra 
observations are collected by an international network of extraordinarily 
faithful observers, many of them amateurs, and each with many years of 
experience.  About 35 observers contribute to the preliminary Ra which is 
available on the 2nd of each month following the observed month.  About 100 
observers contribute to the final Ra values.  The counts are made visually 
with a variety of suitably protected-telescopes.

Final values of Ri appear in SGD, in the IAU Quarterly Bulletin on Solar 
Activity, and elsewhere.  They usually differ slightly from the provisional 
values.  Final American numbers, Ra, are available by the 15th of the month 
following observation, and after collection of all observer reports.


The smoothed relative sunspot number is defined as:

 R(12) = 1/12[Sum(n-5 to n+5) of Rk + 1/2((R(n+6) + R(n-6))]

in which Rk is the mean value of R for a single month k and R(12) is the 
smoothed index for the month represented by k = n.

The predicted sunspot numbers for the 12th month after the latest observation 
are computed using the method of A.G. McNish and J.V. Lincoln [Trans. Am. 
Geophy. Union, 30, 673-685, 1949] and modified using regression coefficients 
and mean cycle values computed for Cycles 8 through 20.  The 90% confidence 
interval is shown by parentheses for each month of predictions in the SGD 
table and by a bar on the graph.  This indicates the uncertainty above and 
below the predicted number.  The predictions are always based on the latest 
observed data available and will change each month as a new observation is 
included in the calculations.  Final International sunspot numbers, as they 
become available, are used in deriving the smoothed data.

The graph in SGD shows the mean cycle, the observations to date of Cycle 22, 
and the 12th month ahead prediction for Cycle 22.  All are shown on the same 
time base, which is that for Cycle 22, beginning with the sunspot minimum at 
Sep 1986.