Sunspots and Propagation
Here comes the sun! A new solar cycle, number 25, has recently begun as I draft this article in late 2021. The sun is coming out of Solar Minimum. What does that mean? Why is it significant to ham radio? And if there is a Solar Minimum, does that mean there is also a Solar Maximum? And what about those sunspots and solar flares? Let’s start here on earth with our ionosphere and then journey to the surface of the sun.
The Ionosphere: The earth’s ionosphere is responsible for the over-the-horizon radio signal propagation that we enjoy with HF and occasionally with VHF. The ionosphere is comprised of layers of the atmosphere in which electrically charged particles are created that collectively have a bending effect on RF signals, directing them back toward the earth from high in the atmosphere. Referred to as skip propagation, multiple skips of RF between the ionosphere and the earth can allow weak signal propagation around the globe. (See HamRadioSchool.com Technician License Course, Chapter 5, Signal Propagation.)
The charged particles in the ionosphere are created by radiation from the sun. The sun emits electromagnetic (EM) radiation of frequencies much higher than radio frequencies, including ultraviolet (UV), X-ray, and occasionally intensely energetic gamma radiation. These frequencies of EM radiation are called ionizing radiation because they may collide with atoms in the atmosphere and strip away electrons from those atoms. When a negatively charged electron is removed from a neutral atom of oxygen, for instance, a positively charged oxygen ion and a negatively charged free electron result. The ionosphere is loaded with these electrically charged ions.
The more ionizing radiation the earth receives from the sun, the greater the density of ions created in the ionosphere, and the stronger the bending effects on radio frequency signals. This is important because the bending effect of the ionosphere also varies with the frequency of the radio signals. As RF frequency increases (shorter wavelengths), the bending effect weakens. The lower frequency HF bands, 20-meter band (14 MHz) and lower, are usually effectively redirected toward the earth by the ionosphere. However, the higher frequency HF bands, such as 15-meters (21 MHz), 12-meters (24 MHz), and 10-meters (28 MHz), will be bent sufficiently to return to earth only during periods when ion density in the ionosphere is relatively great. When this happens these bands are said to “open” for long-distance communications use.
The charged particles tend to be formed in distinct bands around the earth. The bands result from several interacting factors. The increasing density of the atmosphere from space to the earth’s surface interacts with the rates at which free electrons will recombine with positively charged ions to reform neutral atoms, thereby reducing the quantity and density of ions. The depth to which ionizing radiation will penetrate the atmosphere also interacts, impacting the rates at which new ions are created. The net result is the set of ionosphere layers D, E, F1, and F2, each with unique ion densities and characteristics. The layers exist from about 40 to 250+ miles above the earth’s surface.
The Sun: Now let’s venture to the sun’s surface where complex solar dynamics rule, where superheated tendrils of plasma erupt larger than thousands of earths and surge along invisible magnetic loops in spectacular displays of the star’s power.
And where relatively dark sunspots dot the star’s face like an adolescent’s acne.
But those solar blemishes are really important to ham radio operators. Sunspots are correlated with solar energy output. That is, the more active and energetic the sun, the more sunspots there will be. Sunspots are regions of high magnetic activity on the sun’s surface that produce relatively cool interiors, hence the somewhat darker “spot” appearance. But the perimeter of a sunspot will glow much more intensely than other parts of the solar surface, and the net result is an increase in solar energy output with sunspots. In particular, sunspot edges absolutely glow with UV.
Remember, UV is one of those ionizing radiations mentioned above that helps form the ions of the ionosphere. So, with more solar activity comes more magnetically-driven sunspots, and with more sunspots comes more UV radiation to the earth’s atmosphere. The increased UV creates more ions, increasing the density of the ionosphere's layers. And when the ionosphere is dense with ions the bending effects on RF signals is increased. And the greater the bending effect, the higher the RF frequencies that will get directed back toward earth, opening the higher frequency bands for our amateur radio use.
The Solar Cycle: The sun is a creature of habit. It likes to create sunspots (and vary its output and activity) on a regular basis. In fact, sunspot occurrence rises and falls on an 11 year cycle. The periodic peak of sunspot activity is called the solar maximum, and the periodic lull in activity is called the solar minimum. In the previous solar cycle (#24), we experienced a very long and low solar minimum at the end of year 2008. The sunspot numbers increased to a solar maximum in 2013-2014, then decreased again to solar minimum in 2020. Cycle 25 is beginning to ramp up in 2021 and early 2022. The plot below shows the monthly average of sunspot number counts (black line and smoothed blue line) as well as the prediction for coming years as cycle 25 peaks and subsides (red line). The plot covers about 20 years total time, from 2010 to 2030.
The plot also indicates that the approaching solar maximum is predicted to be another relatively low maximum similar to cycle 24. Cycle 24 produced significantly lower sunspot activity than its recent predecessors, and cycle 25 is projected to be a similarly mild cycle. Still, as solar maximum comes on, the higher HF bands will open for lots of long-distance communications activity.
Solar Flares and Eruptions: One last topic on solar activity – solar flares and eruptions. Occasionally the sun hiccups and produces a very intense projection of radiation called a solar flare. When a solar flare is directed toward the earth the very intense ionizing radiation can reach deeply into the atmosphere and create greater-than-normal ion densities in the low D layer of the ionosphere. Interestingly, this has a negative effect on HF skip propagation. What’s going on?
Because of some unique free electron effects in the D layer, the D layer normally absorbs the HF frequencies of the 30-meter band (10 MHz) and lower (40m, 60m, 75-80m, 160m). At night the D layer typically dissipates and allows these lower frequencies to propagate further and skip from the higher F-layer. This is why those lower bands open up at night under normal ionosphere conditions, but may not be very effective during the daylight hours.
However, when a solar flare intensely energizes the D-layer of the ionosphere, the D-layer may subsist for longer periods and may also increase the absorption of higher frequencies such as those in the 20m to 10m bands. When this occurs HF radio communications are severely disrupted, or "blacked out," due to the enhanced D-layer absorption, and we amateurs have little choice but to wait until the conditions pass and the ionosphere gets back to normal so we can play skip radio again. Solar flares tend to ebb and flow with the solar cycle, just like the sunspots, and they can be disruptive of HF communications for several hours to as long as a few days.
The Bottom Line: Now you have a better idea of how the sun affects our ham radio fun. The bottom line effects can be summed up as follows:
The higher HF bands (10m – 17m) will be most effective for skip propagation during the years near solar maximum, occurring on an 11-year cycle. Some of these higher HF bands may not be open during the lower activity portions of the solar cycle.
The higher the band frequency, the greater the dependence on high solar activity for the band to open (for signals to be bent back to earth by the ionosphere).
Sunspots produce increased UV radiation that intensifies the ionosphere and improves skip propagation. Sunspots vary with the 11-year solar cycle, becoming more prominent during solar maximum.
Solar flares and eruptions can temporarily enhance the D-layer absorption of RF, significantly reducing HF skip propagation across the bands.
The current solar cycle (2020 – 2031) is predicted to be a relatively low activity solar cycle, but it will still produce quite adequate conditions for skip propagation on the higher HF bands.
Get out there and enjoy the sunshine, but get in the shack and rack up some contacts too! Good luck! 73
-- Stu WØSTU