Contacter l'auteur / Contact the author

Recherche dans ce site / Search in this site







The short history of the Smoothed Sunspot Number

How to get the correct SSN ?

As you have probably noted, there are different SSN figures, and without more information the amateur cannot know what value to select. 

Indeed, if you don't take care, this is a superb trap for most HF propagation predictions programs, including VOACAP. In fact, all depends on what we speak about, what adds to the confusion !

So let's tackle this problem and resolve it. First, what does suggest George Lane, one of the main VOACAP developer who worked on ionospheric models during 30 years ? George recommends to use the SSN listed at the National Geophysical Data Center available at NGDC FTP website (for users who cannot access that FTP site, here is the GIF equivalent content) because other SSNs are supposed to give erroneous results.

For the small story, only the NGDC's SSN has been used to calibrate predictions from VOACAP, hence George's recommendation. We will see in a one moment why NGDC values have been selected.

SSN : current, predicted, equivalent, effectives, smoothed...


Predicted SSN = 41.7 (the only one source for VOACAP)


SSN = 44.4


Predicted SSN = 33.6 (stated from IPS) :

Predicted SSN = 33.6 (stated from IPS)

GeoAlert-Extreme :

Equivalent SSN = 31 (stated from SEC)

WinCAP Wizard :

SSN = 34 (stated from SEC)

DX ToolBox :

SSN = 36 (in fact calculated for August 29)

IonoProbe :

Current SSN = 36  (in fact the Sunspot number for Aug 30)

Effective SSN = 89 (from real-time interpolation)

HFProp :

SSN = 29

Some "SSN" and assimilated values predicted or calculated for "August 2004" compiled from various observatories and applications on August 30 at 16h30 UTC.

Then, what do you noticed in the field when you are looking for the "good" SSN ? 

SWPC/NOAA is one of the major observatories maintaining SSN indices. However his numbers are slightly different from the one published by NOAA/NGDC. Between both sites the "SSN" difference reaches 60%...

Unfortunately, SSN published by SWPC (formely SEC), and in spite of the advise displayed on their web pages, is used by many VOACAP-based applications (see the table below), and is refered as such in their help file as well, hence the believe that SWPC provided SSN for VOACAP. has well understood the message and suggests to all readers searching for archive and data approved for research to connect on NGDC website. He provides however the "Predicted SSN" interpolated by IPS Radio Space Services, Australia, like do SWPC... But here also the SSN provided does not match with what IPS calls "SSN" ! (see table below). 

 In addition, there are at least four other parameters called "SSN" :  

- NWRA named the "real" sunspot number SSN, which is calculated from optical observations of the sun. 

- There is a sunspot number derived from the 10.7 cm solar radio flux called SSNf (the one used in correlation with SFI). 

- There is a sunspot number derived from fitting an ionospheric model to ionospheric measurements. 

- There is the Effective Sunspot Number SSNe calculated from real-time foF2 observations. 

All these indices are sometimes used as inputs to model the ionosphere in HF propagation predictions programs, but all give different results, and not minors, as the next plot shows. It is thus not surprising that states that "forecasting is an inexact science"..., Hi!

The next plot shows that scientists don't always agree as to what the SSN should be in a particular context. Document NWRA.

The short history of SSN

As you understood, nobody predicts the same SSN as NGDC ! But there is a good reason. Larry Combs and William Murtagh work at SWPC. The observed values that they manage with their colleagues are initially the preliminary values which are periodically replaced with the final values. Official data archive is handled at NGDC and is not the SWPC real-time/near real-time operational responsibility. 

The SSN discrepancy between SWPC and the International Sunspot Index (ISN) exists because of SIDC's responsibility for the official SSN database. This database dates back to the early 18th century. In the late 20th century, SIDC introduced a scaling factor, so measurements of sunspots from the much improved telescopes matched measurements of old. This, of course, will aid greatly in the assessment of the historical cycle records.

On its side, the SWPC SSN number is based on measurements recorded by USAF observatories that of course use relatively new telescopes. No adjustments are made to this SSN calculation. 

The prediction issue is more confusing. There are several methods to predict sunspots, many of them being available on the web. Certain people like one method over another; consequently, it is impossible to agree on a "one for all" method. 

The sunspot prediction established at SWPC is officially sanctioned by the International Space Environment Service (ISES). ISES, with representatives from several different countries, is a permanent service of the Federations of Astronomical and Geophysical Data Analysis Services (FAGDS). It would be considered the "official" forecast. But some will argue that it is not the best and opt for a different method.

This ISES Cycle 24 forecast was the outcome of an international meeting gathering a panel of 12 scientists who met to consider a number of predictions (including the McNish-Lincoln smoothing function used for the SSN estimation) of the profile and amplitude of Cycle 24.

Precursor techniques which use cycle behavior in the declining phase of the previous cycle as a predictor of the next cycle were a big consideration.

The predicted Cycle 24 minimum and maximum were established and graphed by a curve fit which smoothes between the most recent observed values and the remaining forecast values, as shown the graph displayed at left prepared by David Hathaway from NASA/MSFC.

To check : Sunspot cycle and predictions (NASA/MSFC)

There have been a few revisions, mostly because the cycle maximum prediction of 160 was considerably off. But the prediction for a March 2000 maximum date was very good so the overall curve did not change much. SEC astronomers will be working with the ISES community again soon for another update. 

Note that the 24th solar cycle began on 11 December 2007 with the appearence of a magnetic knot at the solar surface, followed on 3 January 2008 with a reversed-polarity bipolar sunspot group.

The NGDC uses a more pure McNish-Lincoln statistical prediction technique that keys on the date of solar minimum. This method is quite popular, but by the authors own admission, it is a better near-term solar cycle predictor (1 year). So even using NGDC value, we can expect some minor fluctuations for short or mid-terms predictions. Indeed, theoretically SSN represents predictions for what we call the "predicted SSN" or simply the SSN. The smoothing is usually over time periods of about 6 months to one year or more. Knowing this, both the daily and the monthly values for the sunspot number as displayed for example at will always fluctuate around predicted numbers.

Back to VOACAP settings

Back to the review of VOACAP-based propagation prediction software

Back to Radio amateur activities


Back to:


Copyright & FAQ