Solar Cycles
When I was first licensed about 6 and a half years ago I remember one of the instructors telling us we were so lucky, we were about to start the peak period of a sun cycle and it was such a great time to be a ham. I remember him saying that any random length of wire or even a barbed wire fence could be a decent antenna during the best part of a solar cycle.
Solar activity can aid or hamper HF propagation, and the info I’ve gathered for tonight came from an article prepared by ARRL Laboratory Technician Mike Gruber, WA1SVF, and ARRL Senior Technical Editor Dean Straw, N6BV.
First, let me state that the interactions between the Sun and our Earth are incredibly complex. Even scientists who have studied the subject for years do not completely understand everything that happens. Here is some general background about how the Sun affects radio propagation here on Earth.
Our Sun radiates an almost unimaginable amount of energy into space. The Sun emits electromagnetic radiation of all kinds, ranging in frequency from below HF all the way to the X-ray region. Much of the energy is emitted as heat. Some solar radiation ends up here on Earth, providing the energy needed to sustain our HF radio propagation.
The Sun also is constantly ejecting material from its surface in all directions into space. This makes up the so-called solar wind. Under relatively quiet solar conditions the solar wind blows around 200 miles per second taking away about two million tons of solar material each second from the Sun. The density of the material in the solar wind is very tiny by the time it has been spread out into interplanetary space. However, even such a low density of solar particles can have immense effects here on Earth.
For years scientists have used different filters on their telescopes to observe aspects of solar activity. Starting in the 1930s, observations began at radio frequencies and now we have satellites that specialize in watching what happens at the Sun and in space.
One of the best gauges of overall solar activity is the number of sunspots seen on the Sun's surface. Sunspots are not really dark, but appear so only because the surrounding surface is hotter and brighter. A large sunspot can be up to 80,000 miles in diameter.
The study of solar activity began around 1750. Long-term sunspot activity varies in cycles. On average, the number of sunspots reaches a maximum every 11 years, but the period has varied from 7 to 17 years. The first cycle to be completely and scientifically observed began in 1755; we know it as Cycle 1.
We are now at the very end of Cycle 24, but we could have also crossed into the very beginning of Cycle 25 as the end of cycle 24 was projected to be mid 2018.
Something to keep in mind is that within the larger solar cycles solar activity also follows a repeating 27-day cycle following the sun's rotational period.
Cycle 23 – Wikipedia states that cycle 23 lasted 11.6 years, beginning in May 1996 and ending in January 2008. The maximum monthly number of sunspots averaged over a twelve-month period during this solar cycle was 120.8 (in March 2000), compared to a minimum of 1.7 at the lowest point. A total of 805 days had no sunspots during this cycle.
Cycle 24 - The current solar cycle began on January 4, 2008, with minimal activity until early 2010. Even with all the hype in my late 2011 ham classes, Cycle 24 is on track to have the lowest recorded sunspot activity since accurate records began in 1750. The cycle however featured a "double-peaked" solar maximum. The first peak reached 99 sunspots in 2011 and the second in early 2014 at 101 sunspots. As mentioned some scientist felt that Cycle 24 will end by mid-2018 but other predictions are that that new cycle will begin in 2019.
So sun cycles last 11-12 years, they often have a single peak in the middle of the cycle, but this last cycle # 24 had a double peak 3 years apart. We’re currently at the tail end of a cycle, so technically it’s the most challenging time in any solar cycle for the amateur radio hobby.
How Does Solar Activity Affect the Radio Bands
Long-time Hams have found that the upper HF bands are reliably open for propagation only when the average number of sunspots is above certain minimum levels. For example, from mid-1988 to mid-1992 during Cycle 22, the sunspots stayed higher than 100. The 10-meter band was open almost all day, every day, to some part of the world.
Right now, few if any sunspots show up on the Sun, and the 10-meter band was rarely open. Even 15 meters, normally a workhorse DX band when solar activity is high, is closed most of the time during this low point in the solar cycle. Sunspots are associated with increased UV radiation. UV acting on the ionosphere is what makes radio propagation exciting on the upper HF bands.
But, wait a minute. Elmers told me several years ago, when sunspot numbers were really high, sometimes conditions were still really bad. What gives?
Well the Sun is a very large, very hot, thermonuclear ball of flaming gases. The Sun and its effects on earthly propagation can be described in "statistical" terms, and they can be averaged to make predictions, however, you may experience vastly different conditions on any particular day compared to what the 11-year average would suggest.
It’s like the weatherman, they can make predictions based on patterns they see in the ever changing forecast, but we’re not surprised when they are get it wrong and rain shower appears on an otherwise sunny day.
There are three general types of disturbances on the Sun that can affect radio propagation, and they often do so in a negative manner. They are Solar Flares, Coronal Holes & Sudden Disappearing Filaments, and like the weather forecast there's not a lot we can do about solar disturbances.
Solar Flares
Solar flares are cataclysmic eruptions that suddenly release huge amounts of energy, including sustained, high-energy bursts of radiation from VLF to X-ray frequencies and vast amounts of solar material. Most solar flares occur around the peak of the 11-year solar cycle. The first earthly indication of a huge flare is often a visible brightness near a sunspot group, along with increases in UV and X-ray radiation and VHF radio noise. If the geometry between the Sun and Earth is right, intense X-ray radiation takes eight minutes to travel the 93 million miles to Earth at the speed of light.
The sudden increase in X-ray energy from a large flare can immediately increase RF absorption in the Earth's lowest ionospheric layers, sometimes causing a phenomenon known as a Sudden Ionospheric Disturbance (SID). A SID affects all HF communication on the sunlit side of the Earth and signals in the 2 to 30-MHz range may disappear entirely. Even background noise may cease in extreme cases. When you experience a big SID, your first inclination may be to look outside to see if your antenna fell down! SIDs may last up to an hour before ionospheric conditions temporarily return to normal.
Typically, several hours after a flare erupts at the Sun, particles begin to arrive at the Earth in the form of a plasma, a highly ionized gas made up of electrons, protons and neutral particles, traveling at speeds up to 300 miles per second. This may interact violently with the Earth's magnetic field. Really high-energy protons may even disable satellites orbiting high above the atmosphere.
Another possible effect of a high-energy particle bombardment during a flare may be high absorption of HF signals propagating through the polar regions. This is called a Polar Cap Absorption (PCA) event and it may last for several days.
Coronal Hole
A second major solar disturbance is a "coronal hole" in the Sun's outer layer (the corona). Temperatures in the corona can be more than four million °Celsius over an active sunspot region but more typically are about two million °Celcius. A coronal hole is an area of somewhat lower temperature. Solar-terrestrial scientists have a number of competing theories about how coronal holes are formed. Matter ejected through this "hole" becomes part of the solar wind and can affect the Earth's magnetic field, but only if the Sun-Earth geometry is right.
Statistically, coronal holes tend to occur most often during the declining phase of the 11-year solar cycle and they can last for a number of solar rotations. This means that a coronal hole can be a "recurring coronal hole," disrupting communications for several days about the same time each month, for as long as a year or even more.
Sudden Disappearing Filament
The Sudden Disappearing Filament (SDF) is the third major category of solar disturbance that can affect propagation. SDFs take their names from the manner in which they suddenly arch upward from the Sun's surface, spewing huge amounts of matter as plasma out into space in the solar wind. They tend to occur mostly during the rising phase of the 11-year solar cycle.
Thus, when the conditions are right, a flare, coronal hole or an SDF can launch a plasma cloud into the solar wind, resulting in an Ionospheric Storm here on Earth. Unlike a hurricane or Nor'easter, an ionospheric storm is not something we can see with our eyes or feel on our skins. We can't easily measure things occurring in the wispy ionosphere some 200 miles overhead. However, we can see the indirect effects of an ionospheric storm on magnetic instruments located on the Earth's surface, because disturbances in the ionosphere are intimately related to disturbances in the Earth's magnetic field.
During a geomagnetic storm, we may experience extraordinary radio noise and interference, especially at HF. You may hear solar radio emissions as increases of noise levels on VHF. A geomagnetic storm generally adds noise and weakens or disrupts ionospheric propagation for several days. Transpolar signals at 14 MHz or higher may be particularly weak, with a peculiar hollow sound or flutter-even more than that which is normal for transpolar signals.
A & K Indexes
Okay, so now you understand a bit more about solar disturbances. When looking online at Solar Propagation reports for Amateur Radio two indexes are often referenced. What do the A-Index and K-Index numbers mean?
Scientists measure geomagnetic activity with a device called a magnetometer. It detects minute changes in the Earth's magnetic field. Since geomagnetic activity can vary with location, a world-wide network of magnetometers monitors it. Two different scales, the A and K Indexes, quantify geomagnetic variations
A Index
The A Index is a daily average of data (from observatories around the world) that reflects the state of the Earth's magnetic field for the preceding 24 hours. The index can be revealing because geomagnetic disturbances due to phenomena such as recurring coronal holes tend to recur at 27-day intervals as the Sun rotates.
K Index
The K Index reflects the instability of the geomagnetic field at Boulder, Colorado, over the last three hours. Such frequent updates can indicate K Index trends. A decreasing K Index is good, especially for propagation paths at greater than 30° latitude. Some VHF operators like to see an increasing K Index, because aurora is possible at K Index values of 3 and greater. Such values also warn that conditions associated with degraded HF propagation were present in Boulder, Colorado. Remember the the K Index is a Boulder measurement, it may not correlate well to conditions in other areas.
The A Index range is from 0 to 400, while the K Index ranges from 0 to 9. Lower numbers indicate better HF propagation conditions.