Most shortwave radio users know that there is a correlation between sunspots and propagation conditions. Sunspots are dark regions on the surface of the Sun, which are cooler than surrounding areas. They occur when the lines of the Sun's magnetic field become twisted. There are more sunspots when the Sun is more active, and produces more radiation which can affect the Earth's ionosphere.

The number of sunspots, and the general solar activity levels, vary over a roughly 11 year cycle. The cycles are numbers, we are currently in Cycle 23. The last solar maximum was in 2000. This cycle will reach a minimum around 2006, when Cycle 24 will begin, which should peak in roughly the year 2010.

The F layer of the ionosphere is caused mostly by ultraviolet radiation from the Sun. During times of high solar activity, there is more radiation from the Sun, and the resulting higher ionization levels in the F layer allow higher frequencies to be reflected. For example, around solar maximum, the 10 meter band (28 to 30 MHz) is frequently open for extended periods of time, and long distances can be worked with rather low power levels.

The D layer is also ionized more during the solar maximum, resulting in more attenuation, especially at the lower frequencies.

Solar flares occur more often during periods of high solar activity, which can disrupt shortwave (HF) propagation.

The Sun rotates approximately once every month. This means that regions of the Sun with large numbers of sunspots and high solar activity will rotate in and out of view of the Earth about once a month, although their activity levels can vary from month to month of course.

Hence, if there is a particularly active region of the Sun affecting the Earth, it will probably be back again in about a month, possibly more or less active than it is now. The same holds true for quite areas of the Sun.

The number of sunspots are counted each day. Since this number can vary quite a bit it is averaged over time, producing the Smoothed Sunspot Number. It is difficult to accurately count the number of sunspots. But there is another way of determining solar activity levels.

The radiation from the Sun is measured at several different radio frequencies. One of these, 2800 MHz, or a wavelength of 10.7 cm, is most commonly used. The signal at this frequency is called the Solar Flux, and there is a rough relationship between this value and the number of sunspots. As the sunspot number (SSN) varies from 0 around 200, the solar flux varies from around 60 to 300. DX Toolbox can instantly give you the current solar flux and sunspot numbers.

By measuring the solar flux, a general idea of the amount of radiation from the Sun that affects the ionosphere can be determined. Higher solar flux levels generally indicate that higher frequencies can propagate. The solar flux can be fed into mathematical models that, along with other information, can estimate the range of frequencies that can propagate over a path, and the expected signal levels.