Solar Terrestrial Indices and HF Radio Propagation

(Originally published in QST Canada, 1992)


Paul M Dunphy, VE1DX
3351 #7 Highway,
Lake Echo, NS
B3E 1C6


Introduction
HF radio waves are propagated over long distances by ionosphere bending of these waves back toward the earth. The degree to which this happens is dependent upon geomagnetic and solar activity. This activity is continually changing. HF radio is still an integral part of commercial and military communication systems. Amateur communication, especially with DX stations, depends largely on HF skywave refraction. To help predict and understand propagation conditions, measurements are provided in the form of solar terrestrial indices. These are the solar flux and the A and K geomagnetic indices. They are broadcast over radio station WWV at 18 minutes past each hour and also distributed over various telephone and computer networks. Amateurs, especially DXers routinely quote these numbers, in an attempt to explain propagation conditions. I've long been intrigued by these numbers and related terminology such as geomagnetic storms, polar cap absorption events, etc.


The Sun . . .
To understand these measurements, we begin with the largest object in our solar system, the sun. It is an ever changing and complex object that only during the last century we are beginning to understand. For example, we now know that the sun has regular, consistent cycles. We know the sun goes through cycles of activity with periods of about 11 years. We know the sun has a 22-year cycle where the magnetic polarity of the solar poles reverse sign. We know the sun is a rotating sphere that completes one revolution approximately once every 27.5 days. We also know that areas near the solar poles rotate slower and take longer to complete one revolution than do areas near the equator.

We also known that there are dark regions on the sun's surface called sunspots. The number of sunspots seen on the sun varies in cyclic patterns. Sunspots are cool regions of the sun, approximately 2000 Kelvin cooler that the surrounding surface. They are regions of strong magnetic fields. These magnetic fields originate deep inside the sun and move outward. When they reach the surface, they cool gasses within the core of the magnetic fields. These fields often curve around and re-enter the sun at a nearby location. At each point where the magnetic field enters or exits the sun, a sunspot is formed. Researchers have determined sunspots are sources of enhanced radiation emissions. This radiation varies in wavelength from Gamma rays down to radio waves.

The sun exhibits a periodic cycle that has a direct result on the population of sunspots over the entire surface of the sun. This is the well-known 11-year sunspot cycle. Observations over many years show us the number of sunspots decrease to a minimum over a period of 6 to 8 years, then rapidly increase to a peak lasting 3 to 5 years.

There is yet another cycle with a period of about 22 years. This cycle is primarily magnetic in nature and can be seen by observing the polarity of the solar poles. Near the minimum of each 11 -year sunspot cycle, the solar magnetic poles reverse sign. This is a rather slow process and often the solar poles have the same polarity before the full reversal takes place. This 22-year cycle affects the polarity of the sunspots that are formed in the northern and southern solar hemispheres. Since sunspots are areas of strong magnetic activity, this solar polarity reversal has the effect of minimizing the number and intensity of sunspots until the reversal has completed.


Solar Flares . . .
If it were just the various solar cycles that affected ionization and propagation, prediction of HF communication conditions would be fairly simple. Unfortunately we have another phenomena that is equally important and much less predictable. These are solar flares. As we shall see, flares are the nemesis of long distance HF radio communication.

Solar flares are powerful explosions in the sun's atmosphere. They occur over complex sunspot groups. It has been estimated a large solar flare can release as much energy as a 10 billion-megaton nuclear device. The most disruptive class of flare is of a type called a proton flare. These flares send out large numbers of high-energy protons that penetrate our earth's atmosphere. They have completely knocked out radio communications over long distances and caused disruptions in ground-to-satellite and satellite-to-ground communication. Other effects include lack of compass accuracy, induction of heavy currents into pipelines, railway tracks, telecommunications cables, and electrical power transmission lines. Flares produce sudden changes in the velocity, density and pressure of the solar wind that have even caused satellites to begin tumbling out of control.

The energy released by solar flares comes from the magnetic energy that has been accumulated over time in an active sunspot region. As terrestrial meteorological storms require pressure gradients, solar flares usually require strong magnetic gradients. This is especially true of the more powerful class of flares known as proton flares. As an active sunspot region develops, the associated magnetic fields intensify. Gradients between opposite poles of the magnetic fields increase. At some point, the gradients collapse, releasing all of the stored energy in a very short time. The sudden release of energy causes intensive explosions in the sun's atmosphere (the chromosphere). Particles are propelled outward from the sun, being accelerated to near light speed within milliseconds. In about eight minutes, x-ray and ultraviolet radiation reaches earth, and high levels of ionization in the ionosphere occur.

Some time later, high-energy solar protons reach the earth. Many of these particles are redirected by the Earth's magnetic field to the Polar Regions where they can penetrate to ground levels. The high numbers of protons at satellite altitudes are called satellite proton events (this is sometimes mentioned on WWV's solar terrestrial report). They can cause satellite communication disruptions and damage satellite systems.

Explosions from flares may last from only a few minutes to many hours. The huge flare of March 6, 1989 lasted for ten hours, compared to the more typical 30 minutes. It was an unusually powerful flare . . . the largest ever recorded.


Polar Cap Absorption Events . . .
Polar cap absorption events (PCAs) occur shortly after a proton flare. We mentioned these flares produce large numbers of solar protons. Within a few hours these high-energy particles arrive at earth. They have an electrical charge and are subject to earth's magnetic field. This field deflects them to the north and south geomagnetic poles. Here, the particles are drawn into the ionosphere at very high speeds. Their energy permits them to penetrate into deep levels of the earth's atmosphere. As they do so, they collide with various molecules causing increased ionization. This prevents radio signals from being reflected by normal ionosphere refraction. Long distance radio communications are severely blocked during PCA events over the Polar Regions. This attenuation of signals is usually confined to the Polar Regions. Major PCAs have caused radio blackouts as close to the equator as 50 degrees. Most reach peak levels soon after the flare and may require several days to return to normal.


Geomagnetic Storms . . .
The Earth's magnetic field has two poles. The lines of force radiate outward from each pole and connect over the equator. The pattern is similar to that illustrated by a bar magnet and iron filings. The solar wind has a strong affect on the shape of the earth's magnetic field. The solar wind is similar to winds on earth, except that it is created by energy from the sun. Just like the earth's winds, solar winds gust and fluctuate in speed. The earth's magnetic field is flexible and reacts to increased pressure by compressing inward and to decreased pressure by expanding outward. When the characteristics of the solar wind change rapidly, the geomagnetic field can become disturbed. This often results in the generation of electrical currents in the ionosphere. The greatest effect on the ionosphere is over the auroral zone. HF radio signals can become so garbled as to be completely unintelligible. Rapid fading of HF signals caused by auroral activity is called auroral flutter. The amount of ionization in the ionosphere over the auroral zone is often strong enough to absorb all radio signals that pass through that region. Polar blackouts are by definition confined to the high latitudes.

Lower latitudes are generally better off during geomagnetic storms. They experience less fading, less absorption and less flutter. They do not escape all the effects. The MUF of the F2 region decreases everywhere. Also the lowest usable frequency (LUF) almost always increases. The combined effects of decreased MUF and increased LUF narrow the usable HF spectrum. At times the F layer becomes almost unusable for HF communications. For those willing to try, the best chances are via CW. Voice communications are very unreliable, unintelligible and suffer severe distortion and fading by the time they reach their destination.


Solar Flux . . .
One of the wavelengths of radiation that penetrates the atmosphere down to ground levels is the 2800 MHz band (10.7-cm). The intensity of noise from the sun at this wavelength is measured daily at the Algonquin Radio Observatory in Ottawa at 1700 UTC daily. This solar flux is used as an indicator of solar activity. The solar flux is measure in solar flux units. One solar flux unit is equivalent to 10^-22 watts per square meter per Hertz. This number can vary from values below 50 to values greater than 300. Values in excess of 200 occur during the peak of the 11-year solar cycle. The solar flux is closely related to the amount of ionization taking place in the F2 layer. High solar flux values generally provide good ionization for long distance communications at higher than normal frequencies. The maximum usable frequency (MUF) during periods of high solar flux can exceed 50 MHz, providing long range communications for 6-meter operators.

The solar flux is dependent on the number of sunspots. Increases in solar flux indicate the growth of sunspot areas on the sun, while decreases in the solar flux indicate the disappearance of sunspots. Since the sun rotates with a period of 27.5 days, the number of sunspots visible on the sun fluctuates with about the same period. They rotate out of view and reappear on the opposite side of the sun about 14 days later. This cyclic pattern is easily correlated to the solar flux.


Magnetic A Index . . .
The geomagnetic A index represents the severity of magnetic fluctuations occurring at local magnetic observatories. The A index varies from observatory to observatory, since magnetic fluctuations can be very local in nature. The estimated A index reported by WWV is derived from magnetometers in Alaska, Canada, Colorado and England. It is calculated from the eight daily K index readings (see below). During magnetic storms, the A index may reach levels as high as 100. During severe storms, the A index may exceed 200. Great storms may succeed in producing index values higher than 300.


Magnetic K index . . .
The geomagnetic K index is related to the A index. Comparing the H and D magnetometer traces (representing the horizontal and declination magnetic components) generates intermediate numbers to reference to quiet day curves for H and D. Each UT day is divided into 8 three-hour intervals, starting at 0000 UT. In each 3-hour period, the maximum deviation from the quiet day curve is measured for both traces. These results are input to a quasi-logarithmic function that yields a K index in the range from 0 to 9. The K index is useful in determining the state of the geomagnetic field, the quality of radio signal propagation and the condition of the ionosphere. K index values of 0 and 1 represent quiet magnetic conditions and imply good radio signal propagation. Values between 2 and 4 represent unsettled to active magnetic conditions and correspond to less impressive propagation. K index values of 5 represent minor storm conditions and are usually associated with fair to poor propagation. K index values of 6 generally represent major storm conditions and poor radio propagation. K index values of 7 represent severe storm conditions and are often accompanied by radio blackout conditions. K indices of 8 or 9 represent very severe storm conditions and are rarely encountered.


Conclusions . . .
The understanding of how the sun affects radio communication is a complex topic. We still do not know how all the forces interact. In spite of the research, we often have good propagation when the solar flux and geomagnetic indices indicate it should be bad, and vice versa. Astronomers and geophysicists continue to improve the quality of propagation forecasting. However, like weather forecasting on earth, prediction of HF radio conditions is likely to remain an inexact science.


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