SSN or the short history of the Smoothed Sunspot Number

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

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.

 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 HFradio.org 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 18^th century. In the late 20^th 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