Sun Spot Number (SSN)

Determination of ionospheric characteristics related to HF propagation requires knowledge of the prevailing levels of solar activity (ITU-R P.1239-3). The Sunspot Number (SSN), quantifying of the number of dark spots visible on the Sun’s surface, has historically served as the primary proxy of solar activity (Clette et al. 2015). Records of SSNs date back over 400 years, providing a valuable insight into the sun's quasi periodic 11-year cycle of activity. Superimposed onto this cycle are shorter term variations that can result in large fluctuations in day-to-day values. More recent research suggests that the 11 year cycle is itself modulated by the interaction of two solar dynamos, accounting for the fluctuations in the level of activity observed during each cycle (Zharkova et al., 2015).

The figure below illustrates how daily values (yellow) may be averaged over month (blue) and monthly smoothed (12-month) (red) periods, eliminating complex short term variations to yield a more predictable indicator of solar activity. The preferred ionospheric metric when determining the critical frequencies of the various layers and the MUF factor M(3000)F2 is a 12-month running mean sunspot number, R12 (ITU-R P.1239-3). R12 values are a function of sunspot values extending at least six months either side of the month of interest (ITU-R P.371-8). (Note that this has the unfortunate side effect that an R12 value for a given month ‘m’ cannot be absolutely determined until ‘m+6’ (six months later)).

Figure 1: Daily and Monthly Sunspot Number (last 13 years)
Source: WDC-SILSO, Royal Observatory of Belgium, Brussels

SSN Values

In accordance with Recommendation ITU-R P.371-8, this site uses SSN (R12) values presented in the ITU's Circular of Basic Indices for Ionospheric Propagation, derived from SSN data provided by WDC-SILSO, Royal Observatory of Belgium, Brussels.

The appropriate SSN value is automatically selected based on the date specified in the Date Picker. The SSN values are updated each month, current values are shown in the table below;

YearJanFebMarApr MayJunJulAug SepOctNovDec
Sources:, (Retrieved: Fri Sep 1 10:12:40 2017)

Daily Values

While smoothed values are required to calculate critical frequencies of the various layers and the MUF factor, it is useful to remember that the actual value on any given day will almost certainly be different to the smoothed monthly value. Figure 2 below indicates the current daily Estimated International Sunspot Number (EISN) obtained by a simple average over available sunspot counts. This data is not intended for long term use but may provide some insight into the stability of the solar conditions.

The (predicted) R12 value for October 2017 is 18.5.

Figure 2: Estimated International Sunspot Number (EISN)
Source: WDC-SILSO, Royal Observatory of Belgium, Brussels