Smoothed-Sunspot Numbers

Sunspot data are averaged or smoothed to remove the effects of short-term changes. The sunspot values used most often for correlating propagation conditions are "smoothed sunspot numbers". A Smoothed Sunspot Number is the 12-month running average for each of the months specified during the solar activity period of interest. Data for 13 consecutive months are required to determine a smoothed sunspot number.

Each smoothed number is an average of 13 monthly means centered on the month of concern. The 1st and 13th months are given a weight of 0.5. A monthly mean is simply the sum of the daily ISN (International Sunspot Number) values for a calendar month divided by the number of days in that month. We would commonly call this value a monthly average.

A Smoothed Sunspot Number of 10 is typical for periods of low solar activity and numbers between 110 and 130 are typical for periods of high solar activity.

Solar Flux -

An additional method of determining solar activity has been put to use, the measurement of solar radio flux. The quiet sun emits radio energy across a broad frequency spectrum, with a slowly varying intensity. Solar flux is a measure of energy received per unit time, per unit area, per unit frequency interval. These radio fluxes, which originate from atmospheric layers high in the sun's chromosphere and low in its corona, change gradually from day to day, in response to the activity causing sunspots. Thus, there is a degree of correlation between solar flux values and sunspot numbers.

Daily solar flux information is of value in determining current propagation conditions, as sunspot numbers on a given day do not relate directly to maximum usable frequency. Solar flux values are much more reliable for this purpose.

Monthly or daily solar activity indices may be used, but the expected 12-month running average is the best over-all indicator for long-term planning.

If a value is entered for Flux then a Smoothed Sunspot Number will be calculated, alternately if a Flux value is entered a Smoothed Sunspot Number will be calculated. In other words, you can enter the required solar indices in either form you find convenient.