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SSN or the short history of the Smoothed Sunspot Number

Propagation prediction calculated by VOACAP online for the 20m band. The SSN was manually added to calculate the global coverage.

How to get the correct SSN ?

Foreword. VOACAP and other propagation prediction programs based on the same engine use an old and specific data set of the sunspot numbers named “Predicted smoothed sunspot number” to NGDC.

This data are not the historical and classic SunSpot Numbers (SSN as described in this document from NGDC) but a monthly smoothed SSN, both qualifier being essential. So in this review as in all VOACAP-based applications, SSN only refers to the Smoothed Sunspot Number.

In addition to what we have just underlined, as you have probably noted, there are different SSN, and without more information the (radio) amateur cannot know what value to select.

Indeed, if you don't take care, this is a superb trap for most HF propagation prediction programs, specially for VOACAP-based models. 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 ?

Because other SSNs are supposed to give erroneous results, George recommends to use the SSN listed at the National Geophysical Data Center available at NGDC FTP website (for backup purpose, here is a GIF image of this file valid until 2020) : "These SSN figures are based on the Lincoln-McNish smoothing function. These are the sunspot numbers used in the database reduction for the worldwide ionospheric maps used in IONCAP and VOACAP".

As states Georges, only the NGDC' SSN was used to calibrate algorithms and thus predictions generated by VOACAP, hence his recommendation.

So the discussion is closed as the engine has never been updated since its release.

But some well informed readers will arg that the SSN provided by NGDC (and calculated by SIDC) is neither the most accurate nor the data that we should use. Indeed, since July 1, 2015, SIDC scientifically endorsed by the International Astronomical Union (IAU) replaced all data by a new entirely revised data series and assigned them other names (see below). This remark is correct.

However, as explained above, amateurs using VOACAP till have to use the SSN provided by NGDC ! To understand this apparent contradiction, let's review the history of SSN. Explanations.

The variety of data

Looking for the "good SSN" to enter into his VOACAP-based application, what are available data on the web wonders the amateur ? Reply: read the VOACAP News where there is a entry directing the reader to the NGDC's file or read the page titled "Choosing the correct Sunspot Number".

The other amateurs, those who never read manuals and would till ignore this hint, will investigate the web and will be face to a serious problem !

The first introduced in the foreword is on the NGDC website itself. The first entry related to SSN directs the reader to a document titled "Readme: Sunspot Numbers" in which it is written : "The SunSpot Number (SSN) is a commonly used index of solar activity. The daily sunspot number was first introduced in 1848 by the Swiss astronomer, Johann Rudolph Wolf [...] Beginning January 1, 1981, the Zurich relative sunspot number program was replaced by the Solar Influences Data analysis Center (SIDC).[...]".

So we have now two different SSN : the "SunSpot Number" and the "Smoothed Sunspot Number" ! It is really annoying.

But it is not the sole issue. Indeed, without knowing how VOACAP was programmed or without having the specific scientific background, the amateur cannot make the difference between all available SSN and others sunspot numbers : there is the predicted SSN, the equivalent SSN, the current SSN, the effective SSN, without to forget the smoothed, mean, weekly, monthly and others yearly SSN. The amateur is thus face to numerous possible SSN what adds to the confusion.

But monthly, smoothed or predicted value to name some data set are of different nature; they do not have the same meaning and information content.

So knowing that VOACAP was calibrated for a specific type of information (scale of input values, time filtering, sampling interval, etc.), we cannot select a priori a data set and injecting the SSN for a specific month in VOACAP, and expect that the resulting forecast will be correct. At a few exceptions (low solar activity and thus a poor propagation that often does not merit to switch on a HF receiver or a transceiver), the prediction will be partly or totally false.

Here is a list of institutions releasing the famous Smoothed Sunspot Number (SSN). If some websites clearly state that their figure is not the NGDC or SIDC's SSN (like Spaceweather who displays the Boulder sunspot number), other sites supporting VOACAP-based programs claim or at least infer that their so-called "SSN" is the one to use in their rapplication, what is totally wrong :

To read : Why the Sunspot Number Needs Re-examination (PDF), E.W.Cliver, AFRL

Predicted SSN for the current day, Sunspot Watch

SSN : current, predicted, equivalent, effective, mean, smoothed...


13-month total SSN = 63 (the official SSN value)


Monthly mean total SN = 69.7 (the official ISN or RI)


Monthly predicted SSN = 39.2 (the historical source for VOACAP)


Monthly predicted SSN = 39.3 (got from SIDC since 07/2015)


Weekly Predicted SSN = 33.6 (got from SIDC since 07/2015) :

Predicted SSN = 33.6 (got from IPS but no more available today)

GeoAlert-Extreme :

Equivalent SSN = 31 (got from SEC)

WinCAP Wizard :

SSN = 34 (got from SEC, today closed down)

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

These data are calculated for August 2004 and compiled from various observatories and applications to get a point-to-point prediction for August 30 at 16h30 UTC. While SIDC data are the official and the most accurate, VOACAP algorithms having been fine-tuned for the "old SSN", it is recommended to keep using NGDC monthly predicted SSN (calculated until 2020).

SWPC/NOAA is one of the major observatories maintaining SSN indices. However until 2015, their numbers were slightly different from the one published by NOAA/NGDC. The discrepancy between "SSN" published by both observatories reached 60%... Unfortunately, SSN published by SWPC (formely SEC), and in spite of the advise displayed on their web pages, it was 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. Hopefully, since July 2015, SWPC also use the SSN calculated by SIDC. managed by NW7US has well understood the message and for years suggested to all readers searching for archive and data approved for research to connect on NGDC website. Today it only lists the sunspot number but it is not used in VOACAP.

IPS Radio Space Services, in Australia, like SWPC provided the "Predicted SSN" until they also use data provided by SIDC since July 2015. You can subscribe to IPS Mailist to get the SSN by email but it does not refer to the data used by VOACAP.

 In addition, there are two other parameters called "SSN" :  

- 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.

without to forget the Effective Sunspot Number or SSNe calculated from real-time foF2 observations.

All these indices are sometimes used as inputs to model the ionosphere in HF propagation predictions programs based on VOACAP engine, 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!

This plot shows that scientists don't always agree as to what the SSN should be in a particular context. Hopefully, since July 2015, sunspots data series have been harmonized and only SIDC in Belgium is allowed to calculate them. Document NWRA.

The short history of SSN

As you understood, nobody predicts the same SSN as SIDC, not even NGDC that download their data from their site ! 

But there is a good reason. Larry Combs and William Murtagh work for SWPC. The observed SSN values that they manage with their colleagues are initially the preliminary values which are periodically replaced with the final values. Until 2000s, the official data archive were handled at NGDC and it was not the SWPC real-time/near real-time operational responsibility.

The SSN discrepancy between SWPC and the International Sunspot Index (ISN or RI) exists because of Solar Influences Data Center, aka SIDC's (located in Belgium) responsibility for the official SSN database.

This database dates back to the early 18th century. In the late 20th century, SIDC introduced a 0.6 scaling factor, so measurements of sunspots from the much improved telescopes matched measurements of old. This, of course, aid greatly in the assessment of the historical cycle records. But if we refer to the new models, this scaling factor is of course useless, and SSN forecast should taken into account this change.

The SSN released by SWPC was based on measurements recorded by USAF observatories using modern telescopes. No adjustments were made to their SSN calculation. However, like the NOAA-Boulder SSN, the USAF series are meant only for short-term use and are based on a limited number of stations (1 to 5). So in no case they are compatible with long term series like the smoothed SSN

Sunspot observations

Generally speaking, one can also wonder which "sunspot number" is really based on sunspot observations? The few examples that we mentionned above (e.g. IPS, SWPS, etc) are only indirect proxies of the sunspot number, i.e. approximations of the sunspot number based on statistical relations with other indirect indices (F10.7, foF2). In other words, sunspots were not at the base of those "sunspot numbers". In fact the name is misleading.

The solar surface on 4 June 2016 showed no sunspot while SSN = 39.4 for that day. Doc SOHO/MDI.

This means that we need to be very careful and aware to the method used by astronomers when we use  SSN to simulate the radio propagation. If it is not a good news, in contrast it allows us to better understand how professionnal calculate it and why all values are not the same.

Those proxy values contain assumptions and are often based on statistics over a limited time interval. In fact, they were often only created to obtain a daily "sunspot number" value, as the primary sunspot numbers calculated by SIDC were only computed on a monthly basis in the past.

So, they were meant only for temporarily bridging this monthly waiting time for the real value. However, today SIDC produce also a daily estimated sunspot number (EISN) directly based on a sunspot observations. That means that today many of those "quick daily proxies" have become redundant and largely obsolete as they were produced when the official sunspot number was only published once a month.

As shows the picture at left of the solar surface with no the least sunspot, neither EISN or SSN predicted for a specific day will match with the real number of sunspots. So, for that 4 June 2016, the predicted SSN for that specific day was 39.4. But this difference does not call into question the EISN or SSN calculation method. However, in a simulation software like VOACAP, for that day it can be pertinent to set the SSN to 0 and to check what difference there is in propagation chart vs. the default smoothed value. The differences can be important (in other circumstances, using SSN=100 instead of 125 can reduce the signal propagation up to 25%).

If you search for a web site providing the predicted SSN for the current day, Sunspot Watch provides daily and weekly values as well as the radio flux at 10.7 cm and other data related to the current and forecasted solar activity. But in all cases, remember that the SSN listed for a specific day remains a smoothed value, averaged, and thus does not match with the sunspot number. This lead us to deal with the calculation method.

Smoothing method

The amateur can wonder which smoothing or time averaging should be used? By heritage from the early 20th century, SIDC provide a serie named "13-month smoothed sunspot number" (and a copy) calculated from 1749 to now.

It is the series that is traditionally used for defining the times of minima and maxima. Looking on data available on the web, we note that depending on one site to another, they give either the monthly means, yearly means or this 13-month smoothed number.

This leads to different values for the same time, leading to confusion, especially as sites that mirror some of SIDC data often don't mention which series they chose. It is not without reason that SIDC provides a description (info box) for all primary series. On its side NGDC also provide some kind of documentation in the same FTP directory where the data files are archived.

Prediction algorithms

The prediction issue is more confusing. We need to make a clear distinction between short-term predictions (trends over one or two years) and predictions of full cycles and the next cycles.

There are several methods or algorithms to predict sunspots, many of them being available on the web. Certain people like one method over another; consequently, until recently it was impossible to agree on a "one for all" method or algorithm. Between scientists, this situation becomes ridiculous ! 

While trend estimates can be done fairly reliably thanks to the Lincoln-McNish algorithm, long-term cycle estimates are still a field of research (there is no consensus and many failures).

To read : Realiability of the McNish-Lincoln Technique

for Predicting Solar Cycle Amplitude in Timing (PDF) 

by E.Hildner and M.Sue Greer, NOAA, 1990

The sunspot prediction established at SWPC was 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). For years, it was considered by some experts as the "official" forecast. But other scientists argued that it was not the best and opted for a different method. Hopefully, today SIDC solved this issue.

Sunspot cycle and predictions from NASA/MSFC.

The solar cycle 24 forecasts calculated by ISES (cycle 2008~2021) 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.

Note that the 24th solar cycle began on 4 January 2008 with the appearence of a magnetic knot at the solar surface, preceeded on 3 January 2008 with a reversed-polarity bipolar sunspot group.

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 right prepared by NASA/MSFC.

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

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 will always fluctuate around predicted numbers.

But in the meantime, science has progressed and models have evolved as well. So, today we need to forget the possible SSN forecasted by ISES or NGDC and only refer toSIDC.

Bottom line

Up to now there is no agreement among institutions, as those different instances of a "sunspot number" are created for different more narrow purposes.

The sole good news is that from end of July 2015, SIDC released the first end-to-end revision of the sunspot number. Their sunspot numbers now match the raw numbers in modern observations. This means that existing software like VOACAP should be adapted to this change of scale (a warning about this change was broadcasted via different channels).

VOACAP users

As explained above, as far as we know, VOACAP algorithms have never been updated for the new data. As by design, the international sunspot number (ISN or RI) was formerly 0.6 times lower than raw numbers like the NOAA numbers, and working for a US company, George Lane chose to design the VOACAP software for the NOAA scale.

Now, as SIDC suppressed the 0.6 conventional factor in the new recalibrated sunspot number, it might be actually largely interchangeable with the NOAA numbers (as far as the scale compatibility is concerned) and thus directly compatible with VOACAP. So, this requires deeper discussions with the possible new developers of VOACAP program. But as long as nobody wants to take this project in charge, unfortunately SWLs and radio amateurs have till to use the "old SSN" released by NGDC.

At last, as states the VOACAP Online help : "VOACAP Online knows about the correct SSN values. Therefore DO NOT set any value to SSN, unless you want to experiment", what confirms what we explained above.

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