Choosing the correct Sunspot Number in VOACAP

Q: I am a user of VOACAP or VOACAP-derived software (for instance HamCAP, MultiProp, WinCAP Wizard 3 or ACE-HF). I do not know which of the available sunspot numbers (SSNs) I should use in any of these programs mentioned. Many people and major HF propagation sites suggest using daily or monthly sunspot numbers from the SEC (Space Environment Center, or from SIDC, Belgium (

George Lane: In short, use the smoothed International Sunspot Number (SSN) values in VOACAP -- or in any other interface program that uses VOACAP as their calculation engine. Smoothed SSNs can currently be found from the Solar Influences Data Analysis Center (SIDC) of the Royal Observatory of Belgium at: . They maintain the (predicted) SSN figures 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 now VOACAP.

The variation of "daily solar flux" over the days of the month is taken into account by VOACAP through the use of the MUF-DAY tables. The trick is to get the circuit MUF predicted correctly using the smoothed International Sunspot Number from the National Geophysical Data Center. This also requires using the CCIR coefficients (same epoch as the F-days tables) and NOT the URSI-88 coefficients (a totally different solar cycle for the foF2 mixed with the old foF1 and foE). Also, the day of the month should not be specified, as this will force the use of the URSI-88 coefficients.

Method 26 will show the FOT-MUF-HPF values by hour for the month. That is where you can see the expected variation of the MOFs due to daily variations in the solar flux. I wish we had changed the designation of FOT to 90% of the days and the HPF to 10% of the days, as they represent the lower and upper decile range of the expect MOF values over the days of the month at that hour.

To be more elaborate, the solar energy reaching the ionosphere is dependent on the number and size of the sunspots. The effects take two to three days to substantially change the ionization in the ionosphere. The net effect is that on a given HF link the Maximum Observed Frequency (MOF) for the path at a given hour will change over the days of the month. An old rule of thumb was that the MOFs would vary around the monthly median value (MUF) for that hour by ± 15%. In the early 1960's the scientists at ITS in Boulder CO collected a fairly substantial data base over a full solar cycle during the IGY (1958-1963) period.

This is what George Haydon wrote in 1966:

  • An investigation of the distribution of daily values of Maximum Observed Frequency (the MUF at a given hour and day in the month) about their monthly median was carried out. Three points in the distribution over the days of the month were considered; values of the daily MUF exceeded 0.90, 0.50 and 0.10. These points are now referred to as the optimum working frequency (Frequence Optimum de Travail or FOT), the Maximum Usable Frequency (MUF) and the Highest Probable Frequency (HPF). [Davis, R. M. and N. Groome (1964), Variations of the 3000-km MUF in time and space (private communication)]

  • Data used in this study was derived from measurements at 13 stations representing a range of geomagnetic latitudes from 71 degrees S to 88 degrees N. The variation in frequency about the MUF was represented in tables of upper and lower decile values for low, medium and high Sunspot Number. Each table showed values for a given season, local time in hours 00, 04, 08, etc., and each 10 degree of geographic latitude from 10 degrees to 80 degrees, north or south.

  • The study indicated that the distribution of MUFs is wider at night than in the daytime and wider at low latitudes than high latitudes in the daytime. Again in daytime the distribution is wider in summer than winter, except at high latitudes where the reverse is true.

    The sunspot number dependence is weaker, but in daytime the difference between two ratios seems to increase with sunspot number at latitudes higher than 40 degrees and to decrease with increasing sunspot number at latitudes below 40 degrees. The distribution of frequency variation was mostly a function of the foF2 and not the M(3000)F2 (the factor used to convert the vertical incidence critical frequency to the oblique path MUF); therefore, the distributions are assumed valid for any oblique path. [Davis, R. and N. Groome (1965), The Effect of Auroral Zone Absorption on High Frequency System Loss, (private communication)] [Lucas, D.L. and G. W. Haydon, [1966] Predicting Statistical Performance Indexes for High Frequency Ionospheric Telecommunications Systems, ESSA Technical Report IER 1-ITSA 1.]

In 1966, tables of the FOT-MUF-HPF distribution as a function of local time at the transmitter, season, smoothed sunspot number (SSN) and geomagnetic latitude (of the transmit site) were incorporated in the first widely used HF ionospheric radio performance prediction program, ITSA-1 [Lucas and Haydon, 1966]. These are often referred to as the F-Days tables. The value of F-DAY was loosely defined as the probability of ionospheric support (i.e. the fraction of the days that the operating frequency is below the MUF).

The development of IONCAP changed these definitions, as the F-DAYS values were only used for the lowest order mode for a circuit-hour. This is also called the MUF mode and is very carefully computed using convergence of up to 5 iterations. Modified distributions are assumed for the MUFs about each of the higher order modes (up to 20) depending on the layer; Es, E, F1 and F2 [Lloyd, et al., 1978] [Lloyd, J. L., G. W. Haydon, D. L. Lucas and L. R. Teters, [1978] Estimating the Performance of Telecommunication Systems Using the Ionospheric Transmission Channel; Volume I: Techniques for Analyzing Ionospheric Effects Upon HF Systems {DRAFT}, US Army CEEIA Technical Report EMEO-PED-79-7].

The authors of IONCAP often stated that F-DAYS are not used in IONCAP. However, in VOACAP the "F-Day" factor given for each of the user defined operating frequencies was changed to show the value actually being used for the most reliable mode (mrm) for that frequency. This new factor is named MUF-DAY in VOACAP [Lane, 2001].

My paper at the Ionospheric Effects Symposium (Alexandria VA, May 3-5, 2005) happens to be about using VOACAP for frequency assignment for ALE links. Rockwell Collins provided me with nearly 22 days of data on a long path from France to Cedar Rapids (over 7,000 km). I had a real headache as the data was taken in July 2004 and the International Smoothed Sunspot number kept changing each month until Feb 2005. It takes 6 months for the smoothing function to settle in on a final value!

Collins had kept track of the frequency having the highest SNR at each hour for a scan of 20 frequencies. My contention was that ALE systems should follow the circuit MUF as that is where the signal is the strongest. A linear correlation analysis of the measured median best SNR frequency versus VOACAP predicted MUF provided a correlation coefficient (r2) of 86%. That is amazingly high considering the fact that the MUF mode changed from 2F2 during the day to 3F2 at night on that path.

I also looked at the correlation between the measured upper decile and lower decile of the frequencies being used per hour with the predicted HPF and the FOT. There was good agreement but not as good as at the MUF.

Hope this is helpful in what seems like a never ending discussion of sunspot numbers. By the way, ICEPAC was supposed to be a daily prediction program but it seems that work on the program ended long before it was ready to be released. But that is the topic of my second paper.