The normal Sun

Last Updated on September 24, 2024 by John Berry

There’s a positive relationship between the R number, and both the received field strength and Maximum Usable Frequency (MUF). The R number is also known as the Smooth Sunspot Number (SSN). The normal Sun is a massive nuclear reactor. Sunspots – dark cooler spots – occur at mid-latitudes north and south of the Sun’s equator. Every 11 years or so, the Sun cycles from no sunspots to a maximum number of sunspots and back again. For MF and HF operators, the more sunspots, the better the skywave propagation.

Solar cycle of the normal Sun

I’ve shown below the solar cycle and the mechanism that leads to sunspot production.

The solar cycle showing production of sunspots on the normal Sun.
The solar cycle showing production of sunspots. The Sun’s inner and outer layers are shown as concentric circles.

At the beginning of the sunspot cycle, there are no sunspots. The Sun’s magnetic field is aligned north-south. An interaction between the magnetic and electric fields causes the magnetic field to tear. Then, taken to the limit (at the cycle peak), the magnetic field becomes multi-polar, with lines vertical and horizontal.

Sunspot cycle of the normal Sun

Sunspots are cooler areas, but when the sunspot count is at a maximum, the sun is at its brightest, hence its cooking of the ionosphere is greatest. Sunspots appear as pairs, one north and the other south of the equator, slewed apart.

The normal Sun and five cycles of its solar cycle.
Five cycles of the solar cycle (from the BBC Sky at Night Magazine at https://bit.ly/3PW8rwc)

There are four cycles, each with a different period. There’s a 2,000-year cycle, a 200-year cycle, an 80-year cycle, and an 11 (or rather, between 9 and 13) year cycle. The last two are of most interest. I’ve shown the 11-year sunspot cycle above. Over 11 years, the sunspot number varies from minimum to maximum. We are presently in Solar Cycle 25 peaking around 2025. The maximum varies over 80 years from about 100 to about 250.

It’s likely that cycle 25 is the lowest peak. Then the sunspot number peak will rise to reach a peak of 250 again in forty years.

Wolf sunspot number & SSN

Sunspots are counted using a method after Rudolf Wolf, the director of the Bern Observatory in the mid 1800s. The number of sunspots varies day by day. On 9th April 2024 it was 64. In radiocommunications we are interested in the Smooth Sunspot Number or R, as a 12 year rolling average. In fact it’s a 13-month rolling average, but referred to as R12. The R12/SSN is available from space weather sites.

HF operators are also interested in R5, the 5-day rolling average, as a metric to use for day by day predictions.